What broke in my suspension?

The rear torsion bar anchor is the most likely culprit. It doesn’t even have to be rust related, some of the factory welds there were pretty bad to start off with. Although just being a California car doesn’t mean it’s rust free there either, portions of the crossmember have access holes so dirt and animals can get in there. A nest made out of upholstery holds water and can cause very localized rust through.

It’s also possible that the lower control arm pivot tube broke loose in the K frame, but that would have to be a pretty extreme case to completely unload the torsion bar like that. Maybe coupled with a blown out bushing.

My bet for a loud catastrophic failure that wasn’t the torsion bar itself would be the rear anchor though.