Pressure got trapped in the system, rear brakes only, and when they got hot, the pressure got higher.
It did not vent with the car stopped, and the caliper failed.
I've never even heard of a caliper failing like that, it must have been as hot as all get out, and could also have been defective to start with, but blow up??????!!!!!
Replace both rear calipers, I'd not trust them if they got that hot. Replace the rotors, too, they are surely warped from getting cooled by the pads in one spot and air on there rest, plus they've been severely overheated.
Then I would bleed ALL the old fluid out, front and rear, and check operation. Replacing the hoses isn't a bad idea either, but you should only have one wheel lock up from a bad hose, not both, unless there is a section of hose before the "t" to the rear wheels.
I would be tempted to replace the master cylinder, too, just to be safe. I suppose brakes that won't let go are better than brakes that won't work, but still.....
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!