I had the parking brake mechanism corrode and freeze locked, you might want to pull the rear disk and have a look at the parking brake setup.

 

I have had one caliper stick on me maybe 6 months after a brake job. Kind of hard to believe both would start at the same time. Maybe something is kind of blocking the brake line. If brakes get hot enough from sticking the fluid can boil making the brakes because mushy. I have had this happen but on a drum and shoe brake system.