It happens,
I recommend a shooting mitt with rubber padding on the palm. Or you can use firm grip ( spray adhesives) like many high power shooters.
What you don't want to do is to grip the rifle firmly with the support hand to try to correct the problem. The forearm should rest on the support hand, not be held by it.
The problem isn't just with ARs as you can see by the below picture. You can tell the points where I used firm grip to assist in a riggid position without reverting to using muscles.
