I had something similar happen to me a while back. Here is the video to show you what happened.
I was flying at my work picnic and all of a sudden SOLO dips forward and takes off really quickly. i had no control. i hit fly and i had no response, The controller did not say Manual ever during this entire event.
It was windy this day but not bad. i had been flying for about 30 minutes before this happened so im sure the wind wasnt too much for SOLO. Either way as you can see in the video it dips and takes off, SOLO is twisting and turning so much the gimbal can't stay level. then all of a sudden i hear. "RETURN HOME" and then i see SOLO flying up as it gets ready to Return Home. Needless to say i was happy that it was returning but i still have no idea why this happened. 3DR said it was a GPS loss but i had no manual control either so im not really sure what happened that day.
Iddentical issue yesterday, I had flown 5 to 10 min over a big field trying to video our tractor working and it just took off... Pretty soon the return home started working and came back it was at 1% battery when it landed so very close call.