Thanks! That was my initial theory as I figured there was no way the bearing itself could generate that much heat - and keep working.The friction from the drive belt entering and leaving the pulleys can warm them up pretty quick. More load on the mower blades means more horsepower handled by the belt and pulleys which adds to the heat load, as will the heat from the spindle bearings under load.
Add it all together and there is a lot of heat involved, and not a whole lot of airflow for cooling.
I appreciate the confirmation.