Haven't had that problem specifically, though I did have a freee on a loading screen, only once though.
 
And the lack of crouch may have been strange at first, but it does make total sense. To survive as a marine you need to be on your feet constantly, and on the move unless you're in Infestation. For the most part, stillness=death, and even the split second it take for you get get up would cost you badly. If there had been crouch, you might as well have renamed it the 'Insta-Kill Me, Please!' button.