Yeah, the engine imbalance causes the wormhole, but it plays like the asteroid is somehow caught in the "warp field" which I don't think had been technobabbled into existence as jargon yet.
Yes, and to add to the technobabble, inside the warp field, the ship is like a passenger sitting inside a train car. Inside the car (warp field) movement is relative, so if the phasers were operative, they could've been fired and it would've led to the same result as firing the photo torpedoes had. The asteroid is destroyed, and the unstable warp field collapses from the blast, instantaneously returning them to normal space. Since the warp field was unstable, you can take some liberties with its behavior and the behavior of things in it.
At least that explanation works for me.