It could be one of a few things:
1. Overheating. If the gearing was way off, or there was binding then rotor failure is one of the results.
2. Bonded rotor - this is basically where the magnets are glued to the rotor, temps rise and the magnets fly off.
3. Some massive impact - though unlikely considering the lack of marks on the can.
4. Manufacturing Defect - it can happen!
There are others too! Best bet would be to send it back to your Reedy distributor/LHS to see what they say - if it's a warranty job then
.