There are many spirits that seem to be prone to unrest and also to haunting the living. This is because of the way they died and their reactions to that transition. It is not that they are evil... or that they are unable to crossover to the next world. Often they are just lost, or simply choose, to stay close to this world and try to resolve things they left unfinished. Sometimes they feel the need to try and let someone know something, that they feel is imperative, for them to find peace. Below are some scenarios that may keep spirits clinging to this plane and why.
People who have taken their own life seem to linger here. There are many theories for why they do this. Perhaps they realize what they have done and immediately regret their choice and want desperately to return to the world of the living. Maybe they are afraid to face judgement for their suicide and so remain earthbound to avoid or postpone what they feel may be a penalty of some kind imposed by God for what they have done. Suicide is considered a sin in most belief systems. There is another line of thought that it is their pennants to stay and watch all the pain their act has caused their loved ones, or that they see the aftermath and stay to try and comfort or apologize to their loved ones. Some people follow a line of thought that suicides remain earthbound for the length of what would have been their normal life span. For example if a person is destined to die at 60 and commits suicide at 23 then they are stuck here until that normal time of death comes around. Now this would place them in limbo for many years, if this were true. I personally do not think that this is the case. I am more inclined to think that one of the previously mentioned causes apply. PPP has dealt with suicide spirits before and they do seem to be very unhappy or in need of some kind of resolution to move on.
People who have been murdered also tend to stick around for many reasons. Their lives were snuffed out without warning and they are resentful of that fact. If their murderer is still at large, they may want justice, and try to find someone who can hear them in order to make sure, that their families know what happened to them and who did it. Spirits have even named their murderer via EVP communications in the past. They will usually also haunt their murderer if he or she is able to sense, see or hear them. If the murderer can not see or hear them, while they are awake, then they can and do often get to the person when they are asleep through their dreams. No one really ever gets away with murder, as even if they evade the law, they can not evade the spirit of the person they killed.
In cases where someone has vanished and their family do not know what happened to them, they will also go out of their way to make their presence known and let the family know they are now dead. If a murder victim feels that their killer did not get a just sentence for the crime, then they also may stick around to try and get someone to look into the legal fumbles that led to the lesser sentence of their killer. This happened at one of our cases. They will seek out those who they think will listen and might help them. At times the spirits of murder victims can even be very bitter and angry when their attempts at getting someone to hear them don't work. Then they may even behave like a hostile entity just out of desperation for someone to know they are there.
Victims of sudden tragic death:
People who are killed suddenly or accidentally sometimes do not even realize or understand what has taken place yet, as it happened with no warning and happened so fast. They may wander about at the place of their death, watching what is going on for awhile, but usually eventually return home to see what is happening there. Once they realize what has happened then they often still hang around to see how their loved ones are dealing with the whole thing and they often make attempts to make their presence known and to comfort those they love. Sometimes people who die this way were not at all ready to die and will cling to this life and this world for quite some time, until someone comes from the next world to escort them home to where they need to go in the next world. Spirits can not be forced to move on from this plane, as we all have free will. They can also return to this plane to check up on their loved ones whenever they choose to. No spirit is ever truly Earthbound as some people claim.
Non-earthbound spirits that return for a reason or a mission:
Spirits who have crossed over to any other plane can and do return from time to time for specific reasons or because they are on a mission. I have seen this in many cases where a grandparent comes back to see the grandchild they never lived to see in life. I have seen cases, where someone was in some kind of bad situation and a spirit comes back to try and warn them , help them, or even try and protect them in some way from whatever means them harm. I have seen other cases where spirits have come back to actively haunt those who hurt their loved ones still on earth, sort of like a karmic thing. Spirits do participate in Karma, as I have seen this many times.
Spirits whose last wishes have not been honored.
When a person's last wishes are not honored, also sometimes do not rest or move on. For example, a man whose estate is not carried out the way he wanted can be very angry about it and will try to convey his displeasure with the people involved in the estate, who made any underhanded moves to acquire something that was not meant to go to them. Stealing from anyone's estate is not a good idea. Also is someone asks you to deliver a message to someone else, while on their deathbed, you really should honor their request. Most important is making sure that the handling of someone's remains is done in the way that they wanted. I have seen problems come about with spirits who were not happy, with the disposition of their remains, and have come back to haunt trying to get that point across.
People who die under anesthesia or from an unintentional drug overdose:
When a person is under anesthesia or knocked out from a drug overdose their spirits, often leave their bodies before death and wander around a bit. This is what we call astral travel. Now imagine the shock when they try to get back into their body and the body is dead and the connection severed. I am sure this produces a state of confusion and panic, that serves to cause them, to return to all the places they knew best in life, their home, their workplace, whatever, as they are trying desperately, to resume, what they knew as their life. Eventually they figure out what has happened and then will either get busy with their afterlife, or behave much like a suicide spirit, and hang around if they have unfinished issues about their lives. Spirits of the drug culture who die from an overdose are very restless and very remorseful. Even if the overdose was not intentional the reckless disregard for their lives tends to end them up in the same mess as those who did intentionally commit suicide.They will often try to haunt others who are still into the drugs in order to try and let them know that this is not the thing to do. Some will also haunt the people who got them into the drugs, who sell drugs, or who are still aiding and abetting those who sell drugs. The drug culture has given us a whole new group of vengeful and restless spirits that while responsible for their own demise, still see fit to also blame those who led them down that road in the first place. Some spirits do pennants by trying to stop others from following the same road to an early grave, as they themselves did.