Tripping causes a winged creature to stall (even if they can hover), and stalling means they start falling.Sejs said:That's what it was. Right, and if you don't have a high enough maneuverability (i.e. Good or Perfect) or the Hover feat, stalling means you don't meet your minimum foreward speed requirements, and thus you start falling.
From the Rules of the Game (for those that desire the information):
"Tripping Flyers: A creature flying with wings can be tripped. If the attempt succeeds, the creature stalls and falls 150 feet. See Rules of the Game: All About Movement for details (and a few unofficial additional rules about tripping flying creatures).
Creatures that fly without wings (and any creature with perfect maneuverability) can't be tripped while flying. If the creature is still in the air after stalling, it must succeed on a DC 20 Reflex save to recover and resume flying. Otherwise it falls another 300 feet. If it hits the ground, it lands prone and takes falling damage."