It is either something that got stuck in there and chipped the magnet or it was already chipped and just got lodged in between the housing and made things worse. It's easy to say check your motors before applying power but I don't. You can't always hear or see these small things either until they develop heat or something catastrophic happens. Hopefully they send you a new one free of charge. My experience has been pretty good with the few places I buy motors from. They know these things come from china and have a high rate of defects.
It seems that a magnet broke and the debris bloqued the motor, it works now but I would not be happy to have that motor flying since there were 3 magnets loose
i'm placing an order but it'd be a few weeks till the stuff gets here if you can't get anybody else to help ya i can..... crap, i need a couple spares now looking at that.