After doing the final shaping of the new mizzen mast today - cutting the "key" that fits in the mast step, drilling the hole for the halyard block hardware - I performed a quick autopsy on the failed mizzen mast. Using a band saw I sliced through the failed mast every 3/4 inches or so below the break. I found that the breaks in the wood went at least three or four inches below the fracture, and all those breaks were in the wood itself, not the glued joints.
It was interesting that when I picked up some of these slices of wood they simply crumbled in my hand. I think the mast, after 14 years of sailing, had multiple stress fractures throughout the wood near where it passed through the deck. It was worn out and during the hard jibe on a windy day down in Georgia those stress fractures gave way and the mast failed.
Note to self: In ten years or so start building a third mizzen mast before the second mizzen mast fails.