If your center of gravity is slightly further back, the lack of friction on your feet (possibly due to wearing socks on wood stairs) will cause your feet to rocket in front of you, therefore falling on your butt.
That's called slipping though, he said tripping.
Wait. The first 2 don't make sense. How the hell do you fall on your butt when you TRIPPED going DOWNSTAIRS? Unless you are a butt face.-Snoip-
I'm guessing those "stairs" were a donkey richard
Mah foot slips and my butt hits stair. :P