The vast majority of "premature slide lock-back" problems I've seen and/or heard about turn out to be user-related. As denner mentioned, above, some part of the shooting hand (usually the thumb) or support hand is contacting the slide stop lever during the recoil cycle, and causing it to lock the slide back.
Next time you shoot it, make a concerted effort to tuck the shooting-hand thumb down, as far down away from the slide stop lever as possible, and if the thumb of the support hand in anywhere near the slide stop lever, do the same with the other thumb. If you don't experience any more premature lock-backs when you deliberately keep the thumbs away from the slide stop lever, then that was the problem.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)