I tend to think they're almost psychic punches in time. For instance in a hectic or traumatic moment in someone's life, they're going to have a profound psychological and spiritual (I hate to use that word) occurrence. I think some instances those can some how loop or replay for some reason. For instance in my youth, we would go to an abandoned insane asylum quite often and hear metals close, loud banging, old radio broadcasts, and other things that seem indicative of another time period. Although this theory doesn't account for a lot of what the GIS picks up, but some stuff does seem like that. I think there's many types of activity that we would associate as ghosts though. Anyway that's my two cents.