Doesn't have anything to do with speed. A false OA detection usually is caused by a low angle sun getting into the sensors. What makes it seem speed related is because the AC has to be tilted in order to go forward. When it stops due to OA, angle changes to level, and sun no longer appears to be in the way. Remember, unlike the main camera with a gimbal, the OA sensors are fixed to the AC body.
You can get around this by turning off the corresponding OA mode, or fly manually in sport mode which disables all OA.