EDIT: NEVER MIND xD
This was due to a hardware fault. (LMB pushed down on the MMB when it was pushed a little inwards due to dirt).

This hasn't been like this until one of the latest updates.

Basically, when I left click while the action menu is open, it will sometimes choose the action like MMB.

I've checked the key bindings and nothing pertaining to the action menu is bound to LMB.

And it doesn't seem to be just a binding thing, because, they only fire sometimes.

Eg. if I bring up the action menu, scrolls to the "Weapon <pistol>", then hit LMB, then sometimes it'll happen instantly, other times it executes the action after I've fired my weapon a few times.

And something about the scrolling triggers it.
If an action is selected and shooting (clicking LMB) doesn't trigger it regardless of how many times I click, scrolling to another action then back makes it trigger again.

This has caused me great grief when my character will eject or switch to pistol over and over.