Thanks Ben. Good thoughts & suggestions all.

In my case, no new electrical devices have been added to this 20A branch.
I kinda think Mark might have been onto it. What I failed to report was that when I opened the pool and first powered up the pump, the motor was "seized" (only gave a buzzing sound). It had never done this in the 8 years I've lived here. After several attempts, I decided to remove the pump/impeller assembly (as I mentioned). With fairly little trouble, I got the impeller to turn and it seemed to turn pretty easily. Put it back together and I thought (hoped) I was good to go.

However, in line with Mark's suspicion, the bearings may be headed South. My guess is that worn bearings - when they get pretty hot - may try to seize or add load to the motor.