Bad feed out of the mag (popped through the feed lips as the round was being pushed forward , instead of being forward through control of the feed lips until the rim clears the lips to guide the round into the chamber) and when you are cleaning the rifle after each event, pull the mags used apart to CLP clean them as well.

If keeping the mags clean does not solve the problem, then suspect either crap mag, spring worn out, or the back U section of the mag has a cracked section that is allowing the feed lips to spread too wide to allow the round to escape.

Really, to check for any of the problems noted above on a mag, load about half of what the mag will hold, the rasp the bottom of the mag off a table.  If the rounds start to fountain out of the mag, the mag has a problem.

Lastly, don't use the mag as a hold point.  This can cause the mag to be well forward upwards during a cycle, and the angle of attack for the round being stripped out and guided by the feed lips can be so high as to miss the chamber completely (end up with the problem at hand).  So if you do need such a hold point, only the front of the mag well, and not any part of the mag in the well isntead.
Note, same can happen when you are trying to use the bottom of the mag off a bench as a tripod point.