A spirit haunting is caused by a human spirit. Non-Human intelligences are dealt with under "Entity Haunting". There is a very real and very big difference between these two, though the paranormal activity produced by both is very similar.
There are many good examples, as well as some bad ones, of human spirit hauntings. The exact cause is situation specific and cannot be easily generalized. Each such haunting requires its own individual exploration and handling. Consider the unique nature of the human spirit and mind.
Even if two hauntings are identical in reason, effect and location, each human spirit is different (unless it is the same person again -- a clear indication that previous actions taken to rectify the situation have failed). These differences, no matter how drastic or subtle, make each human-spirit haunting require dedicated attention and its own solution.
Of course, it is still possible for a good number of human-spirit hauntings to be remedied quickly and easily. If we go in with an open mind and a flexible approach, it is likely that the problem can be cured within a few hours or over a few visits of a few hours each.
Spirit hauntings can be extremely varried simply by the variance in human emotion and motivation. It is also true that even after we do clear up the problem, residual paranormal activity may continue for a while. Keeping up on the cleansing process is extremely important, as is dealing with the emotional and psychological impact the haunting may have had on those involved.
Psychosomatic hauntings often occur as a result of people having too much emotional baggage or emotional scaring after facing an overly emotional spirit haunting. Care must be taken to prevent this from happening.