I'll bet
Although the OP says reload, I'll bet it was done on the initial loading of the pistol at the start of the stage. "Riding the slide" is a common handling mistake, comes from watching to many reruns of "Magnum PI". The finger on the trigger sounds like another newbie error as well.
I don't like the thumb and forefinger technique because it requires rolling the pistol over into a horizontal position, then , then rolling it back vertically to take an aimed shot, two extra movements that use time. Unless, I guess, you shoot horizontal, gangsta style!.
I'm quicker using the slide release lever, but the hand over, gross motor skills and any gun arguments won me over, and I train and reload in that manner now.
I was taught to grasp the slide at the grasping grooves, over the rear sight, and push the pistol forward, while the off hand comes rearward in an exaggerated movement to strike the gun holding shoulder. This was a time consuming technique that had an eye towards rookie safety on the firing line. These days, I drive the pistol forward, but the off hand rolls down ward to reform my shooting grip ASAP after drawing the slide rearward and releasing it at the rearmost of its travel.