It seems as if you "push" your hind leg backwards, away from under your hips on your frontside carves. This makes the end of the turn slip (you hamper the edge grip by this push).
Try to drive out the turn with your weight over the toes, i.e. get your body over the edge (as opposed to push the board backwards). When your knees/hip get over the edge, your ancles tilt the edge and it'll grip. If you just push backwards, the edge has no tilt and thus no grip, and the radius can't follow the carve -> slided turn.
I really thought quicksand was going to be a much bigger issue in my life when I was little