I had this happen with a client last week.I've seen this a few times where folks deliberately stage the trigger then snatch the trigger at the last to get the break where they want it. The resulting loss of control often results in a miss where they would have gotten a hit if they simply held the sight picture and pulled smoothly through with a nice build and follow through.
He was working out his new CZ custom shop pistol and I had him doing some presentations. His first shots, during the pushout, were falling to the left. when I switched from watching his grip acquisition...I changed his draw stroke... to his trigger finger, I saw that he had the trigger coming smoothly backwards until just before the break, where he stopped it, just before he snatched it to the rear.
I had to physically stop the forward motion of his hands to force him to not save trigger travel for when his hands reach the end of their extension.
I have found that having clients fire the first shots DA, during the push out drill, will often cure a trigger slapping problem when they are shooting SA