Pixhawk entering Failsafe mode from ALTITUDE mode

I’ve encountered a new problem and am looking for some help.

One of our drones suddenly switched into Failsafe mode while I was flying it manually in ALTITUDE mode. This resulted in the loss of RC control. When I realized that it was not responding to the joysticks, I tried switching to another manual mode, but it crash landed while descending at slow speed.

My question is, how can the Pixhawk suddenly switch to a Failsafe mode, in this case LAND (the flight log calls it DESCEND) while it is in a manual mode? Something triggered it but I can’t find the reason.