Any codes, check engine light (CEL)? It's was likely a manufacturing defect; perhaps inadequate clearance between the exhaust cam lobe and the bucket follower over the valve spring. This would keep the valve from fully seating in the head when closed, preventing it from transferring its heat to the cylinder head. The valve then overheats and fails. Although this isn't much comfort, at least the valve head didn't break off and destroy the piston. Additionally, carbon deposit formation in the valve guide may have caused the valve to stick open slightly with the same end result. You would have noticed a skip in engine and the computer would have detected excessive misfires, setting off the CEL. However with the low mileage I'd go with the former and this usually happens to the intake valve. Another factory defect could be that the valve seat might not have been sufficiently hardened, causing valve seat and valve face wear resulting in exhaust gas leakage. The only real way to know is to measure the valve clearances before disassembling the head.
I also want to mention that fuel detonation (pinging, knocking,) can damage exhaust valves from overadvanced ignition timing or excessively lean air fuel mixtures. Too much boost without sufficient fuel enrichment in a turbo car can also do this. Any of these would likely provoke the CEL to come on. This is not an exhaustive list by any means but it may lead you to find out what happened. If I were in your situation I'd want to know too.