Bearing damaged by shaft currents.
I have attached a picture of a bearing removed from a 250 HP 480 VAC motor in service for 4 months without insulated rear bearing and shaft grounding. THis motor was repaired by replacing bearings (rear bearing insulated SKF) and added IMPRO CDR shaft grounding brush and seal.
Evident is the washboarding caused by current discharge through the bearing.
While this can happen with an accross the line starter in a system with poor power quality it is very common with VFD drives especially with long feed lines and inadequate line and load reactors.