If you have a Fly spell active, but have "landed", then you are still flying.
If someone is tripped; the natural reflex is to reach out and catch the ground. In this case, the same reflex would mean your feet does indeed get swooped off the ground, but because of the Fly spell, you cannot actually fall over unless you want to.
Your feet will swing up off the ground, then drop back down under you as you hover in mid-air.
Now, if you had Wings, then it would be an immediate Fly Skill check for hovering. If failed, down you go to prone.
Leaving a threatened square provokes normally, but really has nothing to do with being tripped or flying.