MPH is one big glitchfest. That's why I quit playing it. The way I understand it, the walls only work corretly if you're actually moving. If you're holding still the game says "Hey! He's not moving, so there's no reason to make the walls solid because there's no way he can pass through them!" Unfortunately, it only takes into account if you're the one actually moving your character. If you get hit by something that throws you, like say, Trace's alt attack, then it doesn't respond, so if it happens fast enough there's a good chance you can pass straight into the void and instantly die.
You can also trigger this yourself by backing up to an especially thin part of a wall, charging a missle or the magmaul and firing it at the ground in front of you so that the blast pushes you back a bit. It's all too common to see people abusing this trick in online play.
