Broken valve. Any ideas on causation?

This is on a yz450f.  Everything was running strong until it happened coming off a jump.    Any thoughts on what may have caused this?  From what I've heard it's quite rare.

 

 

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Could you take a picture of the valve stem? Straight on with the break. 

What year?

Sorry can't show a photo of the stem, the components are currently at the metal shop.

 

 

Also, meant to include the year.... it's an 08.

Then I'll go back to assuming fatigue failure. The small loads caused by closing the valve add up, and eventually it fails. If there was too large of a gap between the rocker and valve when the valve was closed, the spring is adding extra pressure, and the valve can slam shut, causing shorter life. If the inside of the break looks similar to sand paper or rough rock, then it was a fatigue failure. 

I'll say its from wear. Valves, guides, and seats often don't wear perfectly round, causing uneven pressure on the seat and torque on the stem at the point where yours is broken. May not seem like much, but cycle it a few million times and you can go past the endurance limit. 

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