People falling over is just funny in general, people even laugh at themselves for doing it (obviously not if there is serious injury but just a general fall).
One incident when i was small (3/4ish) i can only slightly remember but my mum is still embarrased about to this day, i saw an old lady fall over while we were out shopping and i start laughing, quite a crowd gathers to see if she is ok and in a quiet second i shout "mummy shes so funny make her do it again" as you can imagine my mum made a pretty quick exit!!!