i'm doing project about abnormal detection using matlab.

hi i am doing my M.E project in abnormal event detection on road side using image processing toolbox in matlab. i checked my video in the block of people tracking which is given as example in matlab. but i dont know how it detect there is abnormal event in that particular video. i used car accident video as input.people tracking block detect exactly abnormal event in that. but how it detects that there is an abnormal event in that video?? on what basis it tracks there is abnormal event?