I don't know how it happened? Engine moving wouldn't affect it. Nothing should be causing the gears to come out of alignment inside the transmission/gearbox. Unless a shaft is bent, case is cracked or something was in there and got caught in the gears and the plastic one got chewed?...
I'm a little miffed as everything was brand new and I have only broken in the engine. No hard running, no jumps, tip overs, flips, nothing.
Like I said, I will check all other gears, shafts, case etc for damage. If everything looks ok I will rebuild and see what happens.