This is thoroughly annoying. It's the third time I'm playing through this game and it's the same thing:

I hold the right mouse button to zoom/aim and click the left mouse button to fire. At random times (in fights) the shield raises itself which I have to drop by pressing Q in order to shoot at enemies again. It seems to happen in some areas more than others (Black Rock etc.). It's like the game "mistakes" simultaneous right+left click for raising the shield.

How many buttons does your mouse have?
I'm asking because, if I'm not mistaken, the shields are assigned by default to key Q and to mouse "back" button. Maybe you occasionally press this button unintentionally and that makes the game raise the shields.