The leader does a dance on top of the tree to show his greatness to everyone around. If he dances all the way up there on top of the tree, then no one can deny his excellence!
If you don't recognize the leader's greatness he will throw ketsup (けつ) all over you (it's a special mix of ketchup and catsup). And it's ketsup because it looks like blood. The blood he'll spill if you don't look up and clap at his great dancing.