Feel bad to see a Hk broke. Just my thoughts, no bashing in anyway.
1 I recently had some issue with s&b ammo. 1 over charged and several boxes with bad primer in 2 different lots. I am guessing that it may be caused by overcharged round? I am lucky the over charged round did not do any damage to my bolt gun.
2 weaker barrel lug is another possibility. I did not call it a defect because your barrel had over 30k rounds. I would not call that a defect. More of a bad luck.