It may be to make sure their finances and their loved ones are ok or to pass along one final message. Often these earthbound spirits do not remain here for long, once they can complete their goal, they normally pass over. If the deed that they are attempting to finish is complicated they will often contact the living to assist them. In many cases the living are unsure of what the spirit wants and it adds to the length of the time the spirit spends here. Almost anything that a spirit feels compelled to do before it goes to the light can make it remain here until it feels comfortable with how they have left things.
Guilt can also be a reason for a spirit being stuck here.
They may feel like they have left the family or loved ones too soon and uncared for. They feel guilty over taking their life or contributing to their death. The living can also hold a spirit here. By not letting go of them, they can be bound here by our love and unable or unwilling to go on until the living can come to terms with the loss. This happens when people go beyond the normal grieving process and become obsessed with the deceased person.