C'mon guys, this is a technical forum, not a flight school. Just imagine the OP said "...and a door came open in flight and had this effect..." and reserve the righteous indignation for something more worthy.
Compared with what people normally do with light aircraft like this, e.g. fly over gross/out of cg, set off uphill on long, wet grass towards trees, go IMC without any training, etc. I'd say it was pretty mild and done in the true sprit of inquiry. If things had started going pear-shaped, they could have just shut the door.
Anyway, it turned left due to the inverse Coriolis Effect, coupled with a spanwise flow moment caused by boundary layer re-attachment at high Reynolds number. So there.