My experience is 95% of the time the cause of breaks are from overstrikes and prying on a stuck blades. Careful choice of end grain orientation and straight grain (minimal runout) reduces the frequency of this considerably and nearly eliminates the sudden shattering of hafts (such as you often see in baseball) upon contact. I can say this with some conviction because I've had the opportunity to inspect hundreds of broken handles over the past 40 years. Get the nicest piece of wood that you can and don't be shy about shaving it down whatever amount still leaves you comfortable.
Perhaps you'll notice that first class single piece canoe paddles have very thin shafts and blades, have a wonderful amount of flex and yet don't easily break. It all revolves around expertly reading the grain before selecting the blank.
Its the same with hatchet handles, probably all wood hafted striking tools. Even with out over strikes they will eventually break.
In the building trade there was an old saying that holds true "twice as wide twice as strong, twice as thick eight times as strong". So we can remove wood to make things stiffer, but it will just be in one plain.
Some old hatchet heads I have laying around. I will never re-haft these as the milled faces are to worn. I bang on them when I need another wedge to split logs length wise.
The remains of hafts are still in them as they broke. The after break autopsy tells a guy a lot about why. As you mentioned the side to side prying is really bad and will kill a handle the quickest I think.
[/URL][/IMG]
No amount of over built would have saved these hafts. To sharp of a shoulder would have led to an earlier break.
I guess if I made two handles out of a pair of sister billets laying side by side in the same stave and made one handle over built and the other nice and thin, then abused them to see which broke first would be a more scientific way to test my hypothesis. But I have better things to do.
A loose head seems to crush wood fibers and will quickly lead to broken hafts. That's why I pay so much attention to moisture around the head. If I can keep moisture out I can keep the head tight. Sometimes the head will start ringing like a bell before I even notice movement.