The belt wasn't over tensioned. I've installed thousands of steppers and servos over the last 25 years, loss of case bearings has never been an issue, only a consideration when tensioning a belt. As I wrote above, I'm still puzzled as to what caused the bushing failure. Not that it matters.
Overload a screw?? I'm assuming you mean direct coupled and linear forces? Neither steppers or Servos have any sort of integrated thrust bearings other than a Belleville spring washer or similar, this must be installed between the motor and the screw in the form of a DRAC (double row angular contact) bearing.
Anything else you'd care to know about the mechanics of motion control? I'm very well versed on the subject, more than I care to be.

Ron