Anyone's magazine ever fall out while firing?

I've previously never loaded more then 10 or so rounds at a time until today. (mostly because I lack the upper body strength and ability to ignore blistering pain in my thumbs )



I just got the upLula loader that makes loading fun! so while at the range today, I thought, "what the heck why not go for the gold and load all 17 +1 ... just to see how heavy 18 rounds feel."



So I took my first shot (with one in the chamber and 17 in the mag) and after firing and extracting (but before chambering the next round) my magazine popped out and fell on the ground!



I've put 349 rounds through my M&P and this has never happened before. Perhaps I may have not have securely fit the mag in when loading, but I'm pretty sure it "clicked" - and it chambered the initial round without a problem.



Is it a coincidence that this happened the one and only time I fired w/ 18 rounds?



EDIT: I just remembered, I did previously switch the magazine release/catch button to the opposite side because I'm left handed. Not sure that this would make a difference, but thought I'd mention it.