I had a normal flight on first battery and started second battery @16.7 volts. I couldn't get my hdmi signal to goggles after i powered them down after flying first battery but not sure if this is related but it's the first time i couldn't connect. Screen on goggles said HDMI 720p but screen was dark not blue which is normal for no signal. So I flew just using ST screen and LOS in angle mode as usual. Launched fine and headed out in a straight line medium throttle and after a few hundred feet suddenly H banks hard right and is heading for some trees. I release throttle but she keeps going so I flip landing gear switch instead of rth (duh) and she stops! I try to fly her back but she is flying in an arch so I switch to smart mode and she behaves normally. Before I land I check voltage and she's at 15.8? this whole thing didn't last for much more than a minute so that surprises me. I reboot controller and H and now battery is 16.4. Everything looks good so I start her up and do a few maneuvers in angle mode and everything went fine. I'm going to re-calibrate everything. Checked the flight log player and I can watch the H suddenly rotate right with no stick input by me except to instantly try and correct which had no affect. I fly in this area all the time and never had an issue.