This is something you never saw 30 years ago, but now rims are lighter and tire pressures are higher, so I can see where they'd fail that way. Yet another advantage of sew-ups - sew-up rims don't take air pressure stress from the tires/tubes. If one failed, the rim sidewall could collapse by 1/2" but the tire would stay in place and remain fully inflated.

eden or hellosunshine, did you notice the rim expanding (maybe the brakes needing to be loosened) before the rim failed?