Just thought I'd throw this out there just in case anyone else has had this problem/issue... I've never had this happen with any Glocks, Sigs, 1911's, Walthers, Rugers, Colts, M&Ps I've ever owned or shot over the past 40 years.
When I got my new 5" M&P Pro 9mm a month ago, I took it to the range and started shooting. On the first magazine change, the slide "automatically" released and loaded a new round in the chamber. This really concerned me due to my extensive experience with semi-autos and having to rack them by hand or use the slide lock/release. The "problem" continued each mag change. I had also bought a new 4.5" M&P .45 at the same time and did not have a similar problem.
I called Smith and Wesson and was told that the pistol was designed originally for LE and was designed that way. Some of the pistols have this "problem" and some don't. I told him that was a pretty lame answer (some do and some don't). Nevertheless, he said they're happy to fix the problem if you send the pistol to them. Since this goes against 40 years of training on all manner of semi-auto pistols where I use the slide lock to release the slide or rack the slide to load the bullet, I will probably send it back so they can correct this for me.