Outer Bearings Wear ?

[Barman54]

I recently repacked an Older Gentlemans Triple 5th wheel, Travel Supreme, 36 ft.
It was at a Travel Trailer Park.
All the outer bearings were pitted, and had to replace them.
Their races were still good.
There was plenty of grease present, and the prior repack was two years earlier.

What would cause the pitting ?
The inner bearings looked good, and were reinstalled.
I know they were China bearings.

I'm to do mine later this year

Thanks for any insight.