You may get this naturally but if not it's easy to just knock over people.
Tips:
Only men can be knocked over, don't bother with women or children. The old guys are the best as they almost always fall over. You have to run into people at full speed right in the middle of their back to trigger the falling over animation. You can loop an old guy to fall over and over so if you get 2-3 old guys you can knock them over every 15 seconds or so.