Quadcopter flew away, PIX4 on PIXHAWK LITE

My copter (F330, PIX4,PIXHAWK LITE) lost control and flew away days ago… I really scared!!

the situation was like this:
1, I was actually testing the battery life, so, I let the copter to loiter above ground about 10 meters, in POS-CONTROL mode. ( before this, I already flew this quadcopter for several days, even in AUTO mode, RC falesafe RTL)

2, during the loiter, I noticed the copter drifting on yaw direction, about 6 minutes later, it turned to left about 30 degrees.

3, then I noticed the copter was rising and drifting slowly,
4,I was trying adjust the coper position with RC, to my surprise, the copter didn’t compounded to RC, even I pull the throttle to bottom position, the copter continued to rise.
5, be afraid of the copter flying toward the city, I turned off the RC , hoping the RC can RTL, I have tested RC
FALESAFE several times before, and I armed the copter in POSCTRL mode, the GPS locked with HDOC less then 1m,
so, I was hoping RTL could be triggered.
6, unfortunately, the copter took a turn and flew toward the sea very rapidly, disappeared in few seconds.

I believe the battery can only support less than 5 minutes flying, so the copter should crashed into the sea.

I can not understand what was going wrong…

Can you share a log? Otherwise it’s very difficult to find out what happened.