This comment is the most likely cause. The torque required to push the valves down is very small compared to the torque the camshaft can take if its not cracked.Not just aftermarket cams, stock ones too.
I suggest a very close inspection of the break, I think you'll find it cracked on installation and has taken this long to finally let go.
You'd be aware that they'll snap like a carrot if the cam caps aren't pulled down evenly along the cam as well as across the caps.
As only a couple of valves springs are actually working as the cam is pulled down, it's easy to get ahead of yourself with the other easy caps.
Also were the pistons down the bores when the cams were installed?
Very easy to push valves into pistons if you follow the workshop manual and try to install cams at TDC.
I always have the crank passed TDC so there's no chance of valves into pistons.