yeah...if you're crouching you're not only vulnerable from behind from a grab but you are vulnerable from the front from being knocked down by a strike attack. not to mention that you have reduced mobility so unless you are 100% sure that you are in a corner perfectly you can easily be manuvered around and grabbed.
best way to avoid being grabbed is to call in your partner, or take off your gas mask and fall asleep. sleeping for a few seconds is usually preferable to being grabbed, unless you really want more equipment.
-g