With the info given, there is no way to determine what caused the failure. Having said that, Lippert is known for inferior workmanship. Was this the cause? Who knows. When you read the number of issues with Lippert, and compare them to major axle manufacturers like Dexter, and AL-KO you realize that something is wrong. How many, non RV manufacturers are using Lippert axles? I'll bet not many. I think the OP's experience with a reputable shop says a lot. Why is the RV industry stuck with these folks?
The excuse is always that Lippert builds what the customer wants, to the customer specs. Really!!! Who is the professional frame and axle manufacturer, Lippert or the RV assembler. It would be like saying that you want a trailer manufacturer to build a 20' long 4,500# rated frame out of 3" structural channel. That isn't a wise move. Maybe that isn't a good example since Airstream does that very thing. LOL
The frame manufacturer should know the GVW of the frame that they are building and should build it to withstand that weight in a variety of conditions. Obviously they need to build it to the dimensions specified by the RV manufacturer so that the trailer will properly fit on the frame, but they can't deny the responsibility that they have in building a safe product. Let's be honest with each other. It comes down to how much money they can make on an uneducated buyer. Having built many equipment trailers on and off since the mid 1970s, I can tell you that an average RV trailer could have a great frame built under it for about $300-$400 more than what they are currently building and that would include better axles, stronger frame, more GVW so you aren't traveling on the edge of it's capacity and workmanship that would last for generations. Check out an old silver Avion frame and compare to it a Lippert frame and you will see the difference.