I had an over weight friend in highschool who could run really fast. He did this by basically by making his top half fall forward which caused his legs to move quickly to keep him from landing on his face.
I think this is a similar phenomenon where the people tripping up the stairs have an imbalance causing their top half to fall forward. As a result the unconsciously speed their bottom half up to catch the top half from falling.
I had an over weight friend in highschool who could run really fast. He did this by basically by making his top half fall forward which caused his legs to move quickly to keep him from landing on his face.
I think this is a similar phenomenon where the people tripping up the stairs have an imbalance causing their top half to fall forward. As a result the unconsciously speed their bottom half up to catch the top half from falling.