Jock,
It appears to me that you have the web of your hand pressing into the deepest part of the grip. On most bows, that is the "pivot" point and not the center of pressure for the bow. Pressing into the bow grip there will cause the forward rotation.
Try to place the base of your thumb (thenar eminence) down and into the grip more and see what happens. I posted some pictures of A recommended grip in a thread called "grip question" here on this forum. A search for that title will bring it up.
Arne