Had a landing fail this morning after a short flight. Using the auto land feature as usual but this time , just as it touched down it seemed like two of the motors speeded up while the other two slowed down. Tipped it on its side so two props hit the ground and stalled while the other two kept going. I had to pull the "up" toggle right back to kill the motors. Tried it again and takeoff was fine but same thing landing again. So next time I did the manual land. Just brought it down to eye level and grabbed it out of the air while killing the motors. I wonder whats going on?