It is my opinion that, based on a lot of those types of breaks that I have seen, it is not really caused by recoil, but more likely bending stress on the stock. The cracks behind the locks usually continue down towards the frame and break loose the lower "fingers" of the stock. So, the crack will open up when the stock is flexed upwards.

Obviously the root issue is a lack of wood in the head of the stock. But I don't think that recoil is the sole cause of it.

B.Dudley