This has been driving me nuts, seems to have been introduced in the patch before the release. When I use the machineguns, I have to crouch in order to get any kind of accuracy and controllability. Occasionally my player will just stand up. If I strike the crouch button, down he'll go only to immediately stand up again.
I haven't been able to figure out what triggers it, but it sure is maddening.
I have toggle crouch bound to left shift, no bind to hold crouch.