Things do break and bearings can be over tightened or left too loose. Having said that, the quality of bearings used by the manufacturer is important. I have no idea what quality of bearing is used by Lippert, but I do know their reputation for axles and frames isn't stellar. If they installed axles that are barely able to meet the load demands of the trailer, then the bearings are probably going to be operating close to their limits. If the OP installed heavier duty Dexter axles (can still use the lighter springs for ride) they will get better bearings and will probably never have another issue.
Why is it that RV trailer owners accept bearing issues on a regular basis as if it is part of the "deal" when you own a trailer. When was the last time you heard of these kind of issues on a car that has a lot more miles than a TT?
These issues can't all be blamed on the mechanic who repacked the bearings. I'm not saying that bad mechanics don't create some of the problems, but they don't create all of the problems.