Jon, nothing to do with the recoil... but rather that unlike on a 1911 where my thumb rests on the safety, on the Hk, the slide release is in that space, so just naturally rested the thumb on the slide release, thereby depressing it down all the time during course of fire.

 

http://www.personaldefensenetwork.com/articles/handguns/maximizing-the-combat-grip/

 

Look at the grips in the article... you will see what I am talking about.

Maks, its certainly possible i was inadvertently restraining the latch. the ergos of the Mak are quite acute and my hands are large so i wasn't counting on me coming in contact with the slide stop but i'll take a further look.