I was thinking about this a little bit more and I think for the unfired round to be ejected, the barrels must have been dropped way down. An ejector operates at almost the full open breech position - obviously so the shells will clear the fences upon ejection.
One possibility is that the shooter's grip wasn't what it should have been, and in an effort to regain control of the wayward gun just after firing the first barrel, the top lever was inadvertently pushed over, and, as the shooter recovered from the recoil with motion transfering back forward, the barrels dropped and the shells ejected.
This all could have happened without conscious effort I think...