Is the bolt going all the way forward?  The bolt being slightly "out of battery" is the only way I could imagine this happening.  I had an old Gen1 PMAG that would drag it's feed lips on the underside of the bolt and slow it down some.

The problem I have understanding the situation is the concept of the trigger not resetting and the relationship that has to the magazine.  Clearly it must be related if the same malfunction happens in 4 different guns.  Otherwise, I'd say your trigger is the problem.  The trigger should be able to move independently of the hammer once the hammer rotates back enough for the sear surface to engage.  The only thing I can think of is that the magazine is out of spec and inserting too far, exerting force on the bottom of the BCG and causing enough deflection to push the hammer too far back.  If the hammer is "cocked" but the trigger isn't reset, the hammer has to be engaged on the disconnector but not on the trigger itself.  Basically the hammer would be held in a position where it is too far back.  Of course that theory goes out the window if the bolt carrier isn't also locked/stuck back.

I honestly can't come up with a solid reason why a magazine would induce that sort of failure.

Maybe one of the feed lips is hitting the BCG causing it to cant or rotate slightly and the hammer is engaging on the outside edge of the BCG.  Grabbing at straw here.....  still not something I would expect you could duplicate on 4 rifles.

Send to Magpul, get a replacement?