It's multipathing. I'm flying either for fun in the woods among trees, or for work in urban areas with tall buildings.
So I know exactly why it drifts, and can even predict how badly it will have issues depending on where I am (I'm getting quite a bit of experience w/ flying in bad locations).
But, it's disturbing that if it's drifting to the west and I give it full stick east, it only slows its drift to the west.. It does eventually stop and start going east, but usually it's run into something by then. (Or now that I'm used to it, I quickly switch modes and resolve the situation)