DISCLAIMER! The following are most likely House Rules unless someone can point us to a better explaination in the books!
With that being said-
At that point I would do something like this:
YEA|
You say that Y (attacker) Bull Rushes E, and A is an ally of Y.
Bull Rush claims that you attempt to push the defender straight back. Assuming Y made the rolls for a sucessful Bull Rush, I would do the following:
Ask A if he will allow movement into his square, or block E from entering.
If A blocks, then E falls prone in the same square with Y. See my above post in which I would deny both Y and E their dex modifiers. I might allow Y an AoO, since E really did try to move into A's square, but got pushed back into it. AoO would be subject to target % as under Bull Rush. On the first initiative cycle after this fiasco that either Y or E could do something, I would require that it include a move out of the same square.
If A allows their enemy E into his square, then both E and A are denied their dex (a reason why A might not want E in the same square!), but both are standing. A would be able to get an AoO on E for moving into the same square with him, subject to the Bull Rush rules for AoO %. Same as above, the first one who goes has to get out of the same square.
If Y has an additional 10' of rush, and A allows it, all three would end up in one big gaggle in A's original square! Nobody would have dex mods to AC, and I'd apply a -2 circumstance to attacks. A would get the AoO chance, modified by Bull Rush %. Same as above, on their moves everyone must at least physically move out of the square so that we're back to one attacker per square.
Finally, If Y had move than an additional 5', I would let him slam E into the wall (assuming A let them pass through), and apply falling damage (1d6 per 10' of rush) with a Balance check for E to stay on his/her feet. Y, E and A in same square, no dex to AC, -2 circumstance to further attacks, etc. Same as above, on their moves everyone must at least physically move out of the square so that we're back to one attacker per square.
Whew, that's a lot of DM shooting-from-the-hip calls, but unless someone can clarify the rules for Bull Rush, that's how I'd do it.
As an alternative, you could modify the Bull Rush "straight back" and let E move diagonally -
YEA| At start of Bull Rush
__E|
_YA| Let E move either up or down diagonal so you don't have
__E| the three-combatants in one square hassles.
What do you think?