Maybe a little too much or too little trigger finger. Those M&Ps do have adjustable backstraps that also impact where the trigger finger falls ya know.
Or possibly some sympathetic "pull" with other 3 fingers as index finger presses - combined with a little subconscious anticipation of what's coming next.
Virtually, impossible to do, but try to hold sight picture through the recoil. It'll let the bullet get clear of the short barrel without the flinch and put you back on target quicker for the next shot.