Let's say you're in the nu-Universe and hit the "Rewind" button. What happens before the Kelvin is destoryed? What happens before the wormhole opens and Nero's ship pops through? Does the universe suddenly blink out of existence, or does it continue backward?
It continues, of course. Why wouldn't it?
I would think of it as more of a duplicate past. As soon as the alternate universe is created, it gets its own copy of the original timeline up until that point...if that makes any sense.
I've got a better idea: One road diverges into two roads. If cars from those two roads move back along them, they can meet each other when they come to a point before the road has diverged.