Thank you for your reply, I think I have a better idea of what's going down now.
Follow-up questions:
1. Attacker WITHOUT reach, but who has power attack, improved bull rush, knockback feat, and dungeon crasher smacks an enemy who has a wall 10 feet behind it. Attacker power attacks and hits for some damage. He gets a free bull rush attempt from knockback, say that succeeds by 5, so the enemy is knocked back 10 feet. It hits the wall and triggers dungeon crasher for some more damage. Does the enemy provoke an attack of opportunity from the attacker when it is moved out of a square that the attacker threatens?
2. Any tips or tricks to get dungeon crasher to activate more often? You're not always going to have a handy wall 5 or 10 feet behind the enemy. Would flight allow you to crash them into the ground? How about a really big jump? That last one makes me think of the Leap Attack feat from Complete Adventurer...
3. EDITED TO ADD: Also, how do these options interact with tactical feats from Complete Warrior like Shock Trooper and Combat Brute? If I had both ST and CB along with the aforementioned feats/class abilities, would I be able to power attack on a charge on round one and trigger all of the various tactical maneuvers such as Heedless Charge to shift the PA penalty to AC, Directed Bull Rush off my Knockback to force them left or right a square, possibly Domino Rush if I knock someone through another person (this one seems REALLY weird conceptually) and next round Advancing Blows from CB (thanks to the auto bull rush from Knockback,) and if I power attack Momentum Swing also activates?