Arduplane 3.5.2 crash strange surge in airspeed before

I´ve assembled a skywalker with pixhawk for a friend that is making agricultural trials to use with a Tetracam camera. Its been working very well before but a few days ago it crashed. It looks like the elevator came off (its the new T-tail skywalker 1720). Dataflash logs show that before loss of control TECS demanded higher airspeeds. I have looked a few logs before but I cant figure out what is going on here. If someone would have a look it would be great to learn.

What I have looked is this. At around line 651000 it starts to loose altitud and at the same time there is an increase in airspeed (demanded also). By line 658500 pitch and roll start to diverge and from there total loss of control and spin to ground. What is not clear to me is if elevator loss might caused this or this high speed caused elevator loss!.