Wheel bearing hot spots/blue discoloration

While repacking the wheel bearings on our FW I noticed a discolored portion on the inner race of a couple of the inner bearings...there is a corresponding discoloration of the axle spindle where the bearing was sitting (the bearing didn't spin... the hot spot created in the bearing went right through to the spindle)

The only thing I can think of is that the race was spinning on the spindle which it should not. Possibly a small bit of crud stuck between the rollers prevented them from going around until they were dislodged going forward/backwards.