[Arcanox]

I'm guessing either you slightly let go of the button (I've had that happen before, where I was holding it but moved my finger and it "let go" even though I didn't feel the button un-click), or you went over a bump. Regenerative brakes physically cannot remain active if a wheel loses traction; they have to re-apply after the wheel gains traction again. This is why I normally cover my brake pedal when I use the paddle.