Sounds like a Fly Away. I've suffered from those twice. From my experience this happens when you take off near something that causes magnetic interference, like a big metal canister, car, etc. Basically it interferes with gps and compass and once the bird gets a correct reading, it thinks it's somewhere it's not suppose to be and tries to correct itself. Best way to correct from a Fly Away as you see it happen is to switch to ATTI and return home manually.
I understand that flying on ATTI is scary, without any assistance from anything, but knowing how to fly manually can get you out a rock and a hard place.
I found that buying a cheap altitude hold drone is a economic and secure way to learn ATTI flying, w/o risking our more expensive birds.
