One thing I discovered recently that could have been contributing, is that my rear brake seemed to be slightly rubbing my rotor. I was riding on a longer ride a few weeks ago, and it got worse. So much so that I actually removed my rear brake pads to continue my ride.
When I got home, I discovered that one of my pistons on my rear brake was out really far and the other was pushed in all the way. I could push the one that was out, back in using a tire tool, but it would keep popping out. The "pushed in" piston was stuck, causing the other piston to extend too far. I pushed in the extended piston and held it while pump the brakes a bit. This caused the stuck piston to push out. Pushed them both back in, inserted pads and seems to be working fine now.
Anyway... Moral of the story.. Could also be some equipment failure mixed with being old
Check your brakes!