runouw.com

I'm not really 100% sure why it works that way, but I suppose it has something to do with how you already slide down it automatically. Somebody else could probably explain it better than me, though. :3

EDIT: *reads post below and rereads the question* *wonders how self hadn't picked that up*

I'm guessing you live in an area that's typically warm year-round, so I'll just explain that snow is often loose or slippery, so sliding when it's at an angle is pretty common.

If you're talking about the very steep slopes, it's a reference to real world physics. Because the angle is at 15* instead of a 45* angle(which is an average stair block), the laws of physics basically say that you must slide back downward.

No, than there is snow (but i not know really these fizic effects was placed in SM63).