Shredded the timing gear...but how?

[fivefix]

I just rolled over 2,000 miles on a newly rebuilt engine by Richard Fallucca of AER. Have had some problems blowing head gaskets with my Snyder's 6.0 head so I had it milled and installed the third gasket. I was gently breaking everything back in and was travelling at 45 MPH closed to home when the engine simply died. I could tell by the lack of compression when I engaged the starter I had blown a third gasket.

As you can imagine, I was quite shocked to pull the head and find the gasket in perfect order. Chocking it up to a rookie mistake, ( 2 year Model A owner) I reinstalled the head and set about getting it running.

The problem became evident as the timing gear was not turning when we tried to locate TDC. We pulled the timing side cover and everything looked good. It was clear the timing gear was not moving, but the crank gear was. After getting the timing gear to move slightly with a screwdriver, the gear engaged and rotated to reveal what you see in the images.

Would love to hear ideas on how the timing gear stopped dead, allowing the crank gear to shear the teeth right off. Once I got the teeth to engage, the timing gear spun freely so there isn't anything restricting it's movement.