I was traveling on a two lane road at about 50mph and the car applied full strength regenerative braking without warning, quickly slowing down to about 40mph when I took over the throttle. I cant remember if I was using adaptive cruise control but I believe I was. No other beeps or alerts were present (as with AEB events). There was NO other traffic present and it was middle of the afternoon with no weather except a bit cloudy. I have no idea what hazard the car could have been reacting to.