does it lock up at first blush or further in the pedal travel as you slow down?
most likely it's a piston is cocking in the bore or sticking. possibly you've got worn pads causing the issue, but it could be any number of things: MC, warped rotor, etx.
ETA: check the lands on the caliper adapters & caliper to be sure that everything is moving freely and debris free. this includes the retainers as well.
time to pull them apart and service them.
