Some are human ghosts who have not reincarnated. They are stuck in that location because they don't want to leave. Usually because they have something they have not finished, like somebody who has been murdered wanting to stay to try to tell everybody what happened so they know who the murderer was.
Also very common that when things are being moved around and knocked over in the house and people call this a poltergeist. It is common that this is not a human ghost, but sometimes it is a thought-form. Somebody who has had very deep emotional problems for many years, this emotion forms into a very powerful thought form that stays in the location. These can effect the emotions of anybody who goes there. And it can also sometimes move objects around like that.