Is there a frame warranty?
Odd, isn't it, how all the photos of broken frames show steel or titanium frames? Possibly a reflection of the fact that few people here ride aluminium frames and even fewer put hard miles on carbon frames, but it gives the lie to the "steel/titanium is a frame for life" thing.
It was probably overheated during welding, or DN6 isn't quite as heat tolerant as other tubing.