New master cylinder after brake work

My 2001 Nissan Pathfinder is at the mechanic getting new rotors and brake pads…he called me a few hours later to tell me something was wrong with the master caliper. What do you think happened?? My brake pedal was a little lower than before, but everything was working when I drop the pathfinder off!

Help! My mechanic cannot explain to me what happened. I have to go see him on Monday.