Helped a young man who had a 2013 Elantra. The right rear wheel would squeel when the brakes were applied very loudly. Car has 50,000 miles on it.

Took off the rear tires and replaced the brake pads. Left rear had normal wear but the right rear pads were worn down to the metal. There was nothing left of the interior pad. The metal clips were also rusted.

Cleaned the calipers and inspect the caliper pins. Lubricated them. Made sure that the caliper was functioning by hit the brakes, it would fully extend.

Tested the brakes and everything is fine now.

What would cause something like that? It does not make sense. Defective OEM brake pads?

A stuck caliper would cause that

Sticking piston, sticking slide pins, pads stuck in stanchions.  If that's all good, take a look at parking brake.  The mechanical parking brake mechanism that's part of the caliper (if it has it) could cause that if its used, and doesn't fully release.  Apply the parking brake, then release it and make sure both rear wheels will spin freely. 

Along with the other things you and JT mentioned the parking brake got me a few years ago.

This is a good answer.

I haven't worked on many Elantras, but I can tell you that new Fords, specifically the Focus, have issues with the rear calipers because of the integrated parking brake.