Pulled trailers and done my own maintance for to many years. Yearly packing of bearings and break inspection.Pulled wheels and hubs found a inner bearing with two busted places on race.Lucky all the rollers were still in the cage. Last load hauled was six head for five hours.Another reason to check bearings.I have never saw a inner race on a bearing bust before has anyone else ever seen this happen. By the way the spindal was fine and there was plenty of grease on and in this bearing. 

Bearing races can be broken by improper torque, and seating of the bearings when they're installed. If the race is pressed into a boss that isn't perfectly clean, it can be made out of round or slightly cocked within the housing. When a new bearing and race are installed, it's wise to double the final torque while spinning the wheel. This will ensure the bearings are seated. Then the torque is reset to its working value, once again spinning the wheel during the final torque.

Some foreign bearings are of poor quality and are not long lived. Replacing them with a known brand will give you peace of mind. The quality of the wheel bearing grease can effect their longevity. However, since you often pack your bearings and have no over heat indications, this is not likely to be a cause of your problem.

If a trailer is left parked for an extended period, a petroleum based grease can slough off the top part of the race and bearing. This exposes the bare metal to moisture and rust. The rollers and race can rust together, and when they are first forced to move, the rust, proud of the metal, causes an interference and can break the parts.

Good quality bearings, proper torque and synthetic grease should offer a life time of trouble free operation.

No heat indication or pitting . Tightened bearings same as i always do. Bearings was and are Timken with Kendal super blue grease.