5000 miles isn't enough for a fatigue issue to occur.

I'm thinking the rocker was running up against a hard stop at some point.

It could be just that cylinder or valve, it could be all of them.

I'd suggest a dye penetrant test on all the rockers. Cracks start from the outside in, so PT will tell you if there are other failures about to happen.

But more than that, you need to find the root cause of the failure.

R.