I am surprised that it would shear off, I would have thought that the cam would spin first since it is only press fit in place. This may seem like a dumb question but was the flywheel seated properly on the end of the cam? Has it ever spun around under the Novi nut? The center section must have been scored or been struck in some way first to weaken it.
I don't know why it would break like that strictly from regular use short of an unlikely imballance in that flywheel, which would show up more at low RPMS as opposed to higher RPMS, there is no load on that flywheel other then it's own weight. Anyhow it should be an easy fix, a thick washer welded to the outside of the flywheel should solve that from happening, it would be a possible upgrade.