Early production M&P 40 made back before late '07?
S&W identified an improperly heat treated steel insert in some early magazine catches. The inserts apparently weren't hardened right and could become worn and start to release the mag unexpectedly during recoil.
S&W was sending out new mag catches to anyone who called and reported that they were experiencing such an issue in one of the early production guns. The owner's manual contains instructions on how to replace (and reverse) the mag catch on page 26 of the current manual (which can be downloaded
http://www.smith-wesson.com/wcsstore...tol_Manual.pdf ).
Otherwise, S&W would probably be glad to examine your M&P and make the necessary repair for you under their owner's warranty.
Making sure it isn't your grip causing the condition is a reasonable precaution, but there were some small number of mag catches with the improperly hardened steel inserts in them used in some of the early guns, too.