Brand new tube broke like this -- what happened???

If you stuffed the tube into the tire and inflated it, it might have had a small fold in it that got trapped and split.

When you inflate the tube the first time, you should inflate it slowly and only a little bit, massage the tire to release any kinks or folds, then slowly increase pressure until about half way. Then give it another massage and deflate it. Then you can slowly refill it all the way.

That’s unacceptable, this is a manufacturer defect and is no way caused by faulty inflation, which isn’t a thing. If it breaks, it was defective, simple as that.
If you need to do this and this so it doesn’t break, find a better brand.
- Its quite possible to have a pinch in the tube which will fail during inflation. In my experience, thats all brands; a tube simply can't withstand 100 psi at a pinch point. This failure is not that, however. This is almost certainly a manufacturing defect, as metioned.

Looks like manufacture defect.

What brand?

Presumably the process of welding the valve part to the rest of the tube failed for some reason and wasn't spotted by whatever quality checks they do.
Schwalbe! That's why I'm surprised, I've never had issues with them.
- That is surprising, Schwalbe stuff is usually pretty solid.
  Switched to their tyres on one of my previous bikes a couple years ago, and was pretty impressed with the smooth quiet rolling, and puncture resistance. Currently running road cruiser greens with their inner tubes

Always buy more tubes than you think you need. They're a cheap, disposable part with lots of defects, so it shouldn't surprise you when you get a lemon. Return this one for a refund.

Thanks, I'll try for a refund.

looks like two holes opened at the valve stem

No kidding ;)