Crash in Loiter mode

My carbon core cortex (Quad setup with pixhawk 2.1) had a crash a week ago.
The RC combination was Taranis with X8R

1.Took off in STABILISE and did some maneuvering
2.Then switched to ALT HOLD
3.Now in LOITER. It was hovering at this point and I tried accelerating forward. Once the input was given it suddenly crashed. At this point, the throttle stick was at hovering position. But the log shows a sudden dip in throttle before crash whereas the throttle was not brought down in Tx. There was no ERROR notification too.

I need help in solving this issue. Hope one of you can find something.
I’ve attached the flight log link below.

Friend i think that happened due to throttle stick. As in loiter mode the vehicle will hover at 50-55% throttle and at 40-45% started lose altitude. If your throttle goes down below 35 it may crash.

@Sudhir If you plot throttle against Desired Actual in ATT you will see the throttle came down after the copter started to lose control.

It looks like motor 1 lost power/failed/went out of sync.
Here is RCout against desired/actual in ATT