possibly over tightening or even just stress. There are dowel pins under it and it may have not seated down properly and then the bolt was used to pull it down onto the dowel.
Those things break so easily man, that's not surprising at all. I actually destroyed the forward end of the one on my last truck, you can ask crvtec, it's sitting in his back yard lol. You can actually just replace that one part, just gotta make sure the new one has the same clearance as the old one. Otherwise it will score your camshaft.
The other way to get the cam towers to match is to shave them slighty, then align bore the cam holes, or have cam bearings installed to match the cam.
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Living the D-50 lifestyle since 1980
Well it could be. I took this right after I put a new head gasket on, the belt was not stock either was the intake gasket, and there was plenty of other signs that it had been torn down once before.
Also if you look closely at the cam tower labeled 2 you can kind of see another crack. Right above the bolt in the lower left.