I think falling over is all part of it. I once fell really hard and my mate said it looked like I had been choke slammed by a wrestler after, he still laughed though.
Heh, I sat out for 5-10 minutes, until the initial pain subsided. Anyhoo, i've no problem with it being funny, just the quality of person who spontaneously laughs at another's misfortune.
In the spirit of this: Please upload the video