There are many reasons why ghosts (demons) are haunting a graveyard.  The main reason is sin. Sin allows demons to haunt a graveyard.  When the demons take over the land this is called a Legal Ground.  The sin committed brought in a curse .  A Curse is also a legal ground for demons to haunt.

Usually the person who committed the sin must renounce the sin ,ask forgiveness and break the curse in Jesus Christ name.

In the case of  a haunted graveyard you don't know who the person is so you would ask God in the Name Of Jesus Christ to forgive the persons who committed the sin in the graveyard, then break the curse.  Breaking the curse  would cause the demons to lose their legal ground  and can be bound and cast out. 

Many young people are curious and want to visit graveyards to see if they can  contact the dead to see if they really come back. The Bible tells us that we are not to communicate with the dead in Deuteronomy 18 : 9 - 12.  When the spirits are called  it's gives demons (ghosts ) a legal ground to haunt the graveyard.

Another legal ground might be Witchcraft that was performed in the graveyard or someone might have defiled the land knocking over the head stones. Defiling the land in a graveyard would give legal grounds to demons as well. Some one might have put curses on the land in the graveyard or buried metals or put cursed objects in the ground.