Looking at the back of the magazine, there is a protrusion of the follower at the left front of the magazine. It is what operates the Slide Stop Lever when the magazine is empty.
It is very close to the feed ramp, so it does not seem that a round being extracted would be able to yaw enough to operate it. I guess a round being ejected might somehow bounce that way, but that doesn't see likely at all.
It also does not appear that the next round rising in the magazine could get so misaligned that it could operate the lever.
I guess some kind of "inertial recoil" might have cause it, but it would likely be occurring more often if that was the case.
My guess would be that one of your thumbs put pressure on the lever as the gun was recoiling and raised the lever.