I can see how it becomes a nuisance sometimes, especially if you're playing competitively but it happens hardly ever. So it shouldn't really bother people.
Apparently some guys did some research on this and got this:
Tripping in brawl explained.
Many attacks in the game can cause a character to trip, but it just seemed to be happening randomly in videos. Well, a couple people did the research and finally figured it out.
1. Your chance of tripping is extremely low, less than 1%, on a slippery surface like ice it rises to around 1-3%
2. You have a greater chance of tripping the more damaged you are, at extremely high percentages it becomes more likley.
3. Certain moves increase your chance of tripping temporarily, lucas's downtilt, stunning moves (like electricity) and so forth do this. (PS Also elemental damage actually matters in this game, and effects how characters move, how likley they are to be knocked off, and so forth, though not enough to be outwardly noticed)
4. You can only trip if you tap and immediatley release the control stick. So normal moving is fine, but dashdancing, or foxtrotting (repeatedly tapping forward to move instead of dashing) gives you a chance to trip. The trip is triggered upon the release, and it can only happen if you do it more than once in succession. So if you simply reverse direction or dash tap once, you won't trip.
5. When you trip you are invulnerable for the entirety of the trip animation, so tripping cant leave you open to an attack, it just kills your momentum so to speak.
