Flying under a bridge means it could have lost GPS signals. Then it goes into ATTI mode, and if it was windy and/or if it got confused, and since you were very low to the water, the sensors sometimes mistake that for you are inches above the ground, then it automatically goes into landing mode, and thinks it is landing onto ground, when it is landing into water. Doesn't help that under the bridge the lighting conditions are suboptimal, and in the low light conditions the Mavic's downward sensors are not reliable sometimes. It seems like the perfect storm of conditions that caused your Mavic to crash into water. Very tragic. Unfortunately, I've been flying Mavic for a long time, and still I would not have taken that sort of risk that you seemed to have taken in the video.
It is not a good idea to fly under a low bridge, especially where there is nothing but water underneath. If you really wanted to do it, you should have at least turned off downward sensors, the water plays tricks on the sensors sometimes at low altitudes.
)
