It's not just WCP gearboxes. Our vex 3cim ball shifter had a similar failure last weekend. See the attached photo of the 1st stage gear.
https://goo.gl/photos/reZTDqnVX9bdwFac7
I'll buy that it's a motor mounting issue. We may not have had the cims bolted tightly, especially as we were fighting screw lengths and the cimcoder on one. This hasn't helped my engineering distrust of aluminum gears though.