Sometimes when I'm playing Super Smash Bros. Brawl, my character will trip and fall. It's similar to when you step on a banana peel in-game. What causes you to trip?
Tripping During Movement
Tripping during movement is (very unfortunately) COMPLETELY RANDOM. It is, however, caused by your character moving forward on the ground.
There are a few common assumptions made about tripping that are wrong. Facts about tripping established through stats:
Tripping By Attack
Some moves, or some hitboxes on some moves, will cause tripping in the range of a lower damage limit to an upper damage limit determined by the weight of your character. Smashboards regular Mugwhump has made a very amusing guide detailing the specifics of what moves and what percentages will cause characters to be tripped by an attack. The basics are that when you are hit by a move which can trip in that damage range, you enter into a HUARGH state: your character falls down and is forced to either get up or roll. Beyond that range, the character will enter into a WHARRGARBL state: they will leave the ground completely from the impact.