That is an extreme outcome, but rear brake drag can come from the secondary master cylinder not releasing line pressure after application, either due to corrosion seizure or fluid blockage. Ultimately this can result in a rear brake that locks on or overheats. A locked rear brake may cause an accident.
Your photo shows one of the fasteners backed right out, I assumed that was partial disassembly during your diagnosis?
I have travelled this path a bit; my story started with a draggy rear brake, I (foolishly) decided to remove the sliding pins for some lubrication with the calliper in place, and when I refitted the larger pin I somehow managed to cross-thread that so the two pins were no longer truly parallel. I have had plenty of garage experience and the cross-thread was not obvious when re-tightening. When tightened this pulled the back pad into firm contact with the rotor. My only solution was to replace the brake mounting bracket with a second hand part.
I disassembled and cleaned the pistons and seals, and then also did the same with the secondary master cylinder to ensure the one-way valve and cartridge were truly clean. There are some here that will tell you that that is also verboten but it is just hydraulics like the rest of the system. The Honda LBS works great but is unforgiving of skipped maintenance.