Sometimes with roller or tapered roller bearings that have too much grease the rollers won't roll but get bound in the cage and slide.
If a motor that has an axial drive (no belts) has a roller bearing in the output end this will result since the bearing has no load to make the bearing rollers turn.
In this case the proper bearing would be a ball bearing instead.
We have to make certain that the motors we install have the proper bearings for the application or this might result.
Often the cage makes a high pitches screeching noise when this has progressed.
The trailer bearings will always be loaded, however and this should not be an issue.
It is possible that the bearings are over greased and the grease did not clear on initial use and the rollers did not initially turn.
A bearing should not be packed more than approximately 1/3 full to allow the grease to be pushed from the races and not have this problem.
Over forty years I have seem almost as many problems from over greasing than under greasing.
However you pays your money and takes your pick.
Since you said not to give advice as the worrying about preload etc, I won't
The other reason for fretting is the trailer or bearing during shipment or if it is mounted in a high vibration and the bearing sits in one spot while shaking.
This can be brinelling or false brinelling.
Here is a link to the SKF PDF in this subject.
This is a good reason to maintain bearings, but an important thing is to clean and inspect the bearings even when replacing to detect any sign of distress.
This is a major part of my work in preventative maintenance and in determining the cause of any failure or incipient failure.