I believe it was from the mag springs being too strong. I had a few of these early on, but have had none in the last 500 plus rounds using various ammo through my EZ. I may be wrong about the springs, but the problem went away with use...

Edit: I should clarify. The issue I had early on was occasionally (maybe 5 times) the last round in the mag would not load into the chamber. Rather, it would be sitting upright (bullet end up) with the casing caught in the ejection port...as though the round was pushed too far up or up too quickly.