I think this may be due in part to finger placement on trigger, possible recoil anticipation and a factory trigger pull.
Try pulling your trigger finger out a bit, and possibly try to place the hump on the rear bottom of the grip, more toward the center of the palm, than on the muscle of your shooting hand.
A bit hard to explain that, but what I have done is, single hand grip of the gun, and then with the non shooting hand, twist the gun so the hump rotates down into the space between the muscles of your hand.
Takes a bit of time to get the feel of this grip, every time, but should help make your grip a bit more stable.