I agree with the gripping too tightly... but only with your thumbs.
My girlfriend tries to ride the slide stop like it's a second safety (on a 1911) with her left hand. Causes the exact same problem, does it with the M&P too. She's just looking for more stuff to hang onto. Grip hard with your fingers, no grip with your thumbs. Mine are pretty much floating free relaxing both pointing at what I want to break.
For me, my right hand thumb is sitting on my 1911s safety, or where it would be (resting on my left thumb's knuckle). Left thumb is well forward of the slide stop, lots of people creep their high thumbs grip back as they shoot. If you were to give yourself a "thumbs up" your thumb should still be pointing down the barrel of the gun, not straight up into the air (what I mean by the grip working back. Your fingers aren't pointed down anymore, they're pointed forward).