Thanks for the great input.
I agree with the theory that a tube can't simply blow inside a tire. It must have 'gotten out', so to speak.
I think in my case, one of two things happened:
1. This specific tire and rim combination simply is not compatible, and the tire slipped off the rim.
- Maybe too much baby powder during installation made the bead slippery?
2. Bad installation.
- Tube was too large, given the wide rim the tire sits wide leading to a smaller overall diameter.
- Tube then got pinched under the bead of the tire and the air was able to slip under the bead and out in the part of the tube stuck outside the bead. But this wouldn't necessarily cause the tire to come off the rim?
After both blowouts, the tires looked structurally fine - came off the rim clean both times.
Either way, I will not use this combination again, which is a bummer.