StreetDancer, no ryme or reason, could just be bad parts. But, they may have not been packed properly, bad lube product, poor mfg, or poor installation, or overtorqued. Lots of possibilities. It would be interesting to know who mfg. the bad wheel bearings though. See if you can get that info out of the svc. department. Just one more thing to put on the long list of things to watch out for, but may never happen.