I don't think you'll find any hard and fast rules to cover this, so I'd probably go with
1) The target feels the earth start to move under him and easily stops himself from falling in
2) The burrower can try to avoid (1), by using move silently to stealthily burrow under the target. Reflex DC = move silently roll. I might adjust this after some careful thought or playtesting. The burrower needs to move at half speed to move silently, or do one of the things that allows you to move full speed silently.
3) If the burrower end his turn under the target, the target cannot fall in because the burrower is occupying that space. Instead the target is prone in that square. If the target does not end his movement directly below, then he provokes an AOO for moving through threatened area, though I'd give a 50% miss chance for concealment, and no AOO if the target fails his reflex save.
4) This all assumes a medium burrower. I'd have to think more for different sizes.
Good luck!
