If you go down a slide and get dirt on your legs, it must have been the fault of the previous person, aka the person who went down the slide ahead of you. They should've cleaned off their dirty legs before they went down the slide, but they didn't, and now you've got dirt all over your legs too.

Then you realize: the previous person on the slide must've been a centaur (せん). Centaurs track dirt everywhere 'cause they're always galloping around in the wilderness.