theres two threads on it other is here. prob where u seen it
http://twostrokemotocross.com/forum/technical/yz-250-engine-seized-i-think/im assuming poor maintenance, aswell as age of the bike and as you said fatigue, i dont think the piston was ever changed, its a 02 so its a 11 years old with out a piston change . though previous to this bike i had a 1992 rm250 fully restored well to my taste, but i didnt do the engine much was young never changed the piston but maintend well cleaned filter changed oil, ran the correct ratio and never had a problem and my brother had the bike previous with no problems got pics if interested?
and yeh i bought a new barrel
Update.
rebuild kit arrived + new plastics fitted pics soon
about ready to split cases change crank and bearings and the bottom end back in the bike.