Your swing path has to be traveling left through impact & your club face angle is looking left too, so it's square to your leftwards path. To do this the club has to be traveling a little ways right to left in direction (out to in) Doesn't have to be a massive out over the top move to get this starting to happen. Could be your timing was just a little out so both lower body & upper body starting transition at more or less the same time, have the effect of making your downswing a little steep & to the left a ways.
The other thing could be your ball position got a tad forwards to where it usually is, sometimes happens when body alignment slightly open, means the club head gets to impact a ways later in the swing arc when the face angle is looking left of target so got to ball after it being square & the swing path has started to go back inside (as it should) so the path at strike is leftwards, face again square to the leftwards path.
Unlikely, to put the ball too far forward in the swing with the irons, not something that often happens. Would think you were more likely to have just got a little out to in with your timing a little out perhaps.
