The other thing that can cause a failure is improper fit of the wedge to the kerf and eye, quite common. The wedge taper must match the eye and fill all the empty space, (the kerf and any space between the top of the handle and sides of the eye).

Measure the eye width and length, from the bottom and the top. The handle and the wedge, must match the measurements with the wedge installed out of the eye plus a little extra to compress the wood enough to fill the eye tightly at the base of the eye and middle and slightly mushroom the top of the head.

Any empty space or gap is an area ripe for failure. As are crushed wood fibers from forcing an improperly fitted/tapered or too large wedge to the kerf.

It takes just a few minutes to take some measurements from the eye and the handle with the wedge installed and fit the handle and wedge to the eye.

I restore and sell old tools, and buy a lot of hammers, hatchets, and axes. Most of what I buy are broken handle, discarded tools. The cause of failure is often an improperly fitted wedge.