At a guess I'd say the cams broke for lack of lubrication to their
There are oil passages through the gasket to allow oil, under pressure,
to move from the block into the cylinderhead for distribution to the
cam. Installing the wrong gasket could block these oil passages and
after some miles the cam would melt its bearings and seize.
Second possibility: Upon reassembly the cam was not synchronized with
the crankshaft - not correctly timed - allowing the valves to be hit by
the pistons. This happens when the timing chain (between the crankshaft
and camshaft) breaks. Big expensive mess - some new valves, valve guides
and camshaft. This horror would be almost immediately evident when the
engine was first started.
Third possibility (speculative): The timing chain is kept under tension
by an oil pressure actuated ratchet mechanism (or similar). To remove
the cylinderhead the chain must be slackened - tension removed - by
releasing the tensioner. I speculate that it could be that the tensioner
was not reassembled correctly and the chain was untensioned - floppy -
perhaps even to the point of jumping links on its sprocket and so losing
the correct valve tining as described in Second above.
However it occurred, it's an unhappy situation between you and the
repair shop that installed the head gasket. The camshaft didn't just
happen to break independently of the head gasket's replacement, IMHO.