Bike weighs as much as a small car. You sit on the rear brake with its single rotor with poor air flow so it doesn't cool like the front rotors - you may even have panniers to make matters worse.
Brake fluid can begin to boil causing temporary air bubbles.
BTW, brake fluid should be changed every 5 years in my experience.
I'd still remove the pads and clean and lubricate them using Coppaslip. It's a 5 minute job to remove them.