It is believed that ghostly activity takes place due to horrible deaths-where physical/emotional trauma have taken place.
One of the bloodiest battles of WWI was fought in the village of Passchendaele, Belgium, on July 18th 1917 and lasted for months. "Disembodied sounds such as voices, machine guns, screaming and doors slamming (trenches) have been heard in this desolate village." (wiki) Heavy rains turned the battlefield into a muddy swamp making fighting extremely difficult. A number of men struck down by enemy bullets drowned in the thick, suffocating mud. It is uncertain as to the exact number of casualties from both sides, however the numbers are in the hundreds of thousands. Many bodies were never found and a number of those that were are in unmarked graves.