I think we laugh because it just looks funny and out of the ordinary, not because the peson failed. We laugh because the situations and movements that happen, are uncommon. When others fail in a competition that we are also in (like a game or a race) we might feel happy, but that is just because we are happy that we didn't fail, not that the other person lost. We are socialized into a winner/loser society and think there is no other way to play the game of life.

These two feelings get mixed up. When we are happy that we didn't fail, we think that we are having Schadenfreude, which is wrong in my opinion. We are just happy that it didn't happen to us.

Imagine your on a camping trip with a friend. If your friend gets bitten in the butt by a dog you might laugh because it just looks funny. But if your friend is killed by a wolf, you would certainly not laugh. Still, you would be happy that it didn't happen to you.

This becomes obvious when you watch fail videos where people actually get hurt. You might laugh because of the funny situation at first, but when you see that they got hurt, the laughter quickly turns into silence.