You're a lucky guy. All it takes is a momentary lapse, going into "fourth".
Here's how I broke one of mine;
With 3.55s I can hit 65@ 5500 at the top of Second gear. So showing off, I floored it. and about 4/5 seconds later that's cruising speed, so I skipped Third, and stuffed it into "fourth", and Zing; the flywheel tore em'all off.
So what I'm saying is; it ain't how much engine you have or don't have. In this case the throttles were closed, and the rpm was falling.