What is an earthbound ghost? Simply it is a person who had died and have not crossed over to the other side. Earthbound ghosts, why do they still here and cross over. There are several difference reasons.
If they violently died, murder, suicide, or accidental. They might stay here to see justice done, their killer was put behind bars. With accidents, like a car crash, they might stay here to make sure that everyone who survived was going to recover. They feel they could offer some kind of help.
Another reason, which ties into the above reasons, is they don't know that they are dead. If they died in an accident they might not relies that they died. Just like in the movie "The Sixth Sense", the doctor died after a former patience killed him and he didn't know he was dead until the end of the movie.
Unfinished business will keep a ghost here on the earth. They haven't said good-by to everyone yet. What ever the reason to them they can't leave until everything had been completed.
In the TV show "Ghost Whisper" there was a ghost who was afraid to go into the light. He was afraid of what might await him on the other side and he felt safer on this side.