Once upon a time I watched PBS Nova program about string theory. I remember they talked about something along these lines. It's been almost two decades since I saw it so memory is rusty. But it was something along the lines of universes existing in these big "planes" (they visualized them like these big floating membranes) that would vibrate and occasionally smack into each other. When one of these collisions would occur, there was the potential for a new plane to be formed from that collision.

Like you said, that could potentially explain where new universes come from, but not how the other "turtles" got there in the first place.