Lets see what causes a valve to burn:
1. Valve clearance too tight.
3. Possible cam timing off and the exhaust valve opening before the combustion event has completed
4. Shitty initial valve sealing when the engine was new.
5. Carbon deposits on the stems not letting the valve fully close.
I'm sure I'm missing a few possible causes but pictures are worth a thousand words.
Some of the above would have other symptoms before damage was done.