Well I imagine the trucks are up to the task, when you consider that the 3000 boxcars were rated for 50000lbs on similar trucks.
There could be a number of reasons why one could have had a hot box.
Coincidence could explain hot boxes on both cars.
1. Not enough oil in the journal because no body checked them.
2. Dirty oil, or old oil.
3. Not enough packing, or improper packing job.
4. If the cars had even a small derailment (Like one axle) it could shift the bearing on its side during all the bouncing around. And then again not checked after a rerail.