Hey Paul,
Unfortunatly, I'm kinda an expert in exactly what you did. I've done it 3 times before. What happened is that you didn't use loctite on the sprocket bolts (or didn't use enough). One got loose and fell out. Instead of even pressure over 6 bolts, you have uneven pressure over 5 bolts and the next one in line can't take it. So it breaks. Then you've lost 1/3 of your bolts and you have a chain that wants to rip your sprocket & hub apart.

I've been there before, and I tried to blame anyone else I could find. Unfortunatly, just a bolt fell out....... at least that's my guess.