A hole in the piston like you have pictured is usually due to one or a combination of the following:

1.  Too much compression for the octane rating of the fuel being used.  (fuel looses octane rating points with age.  )

2.  Ignition timing too advanced.

4.  Spark plug heat range too hot.

5.  Exhaust system has too much back pressure (Spark arrestor plugged, and or Stinger size too small for the power the engine is developing )

6.  Maintaining a constant speed at partial throttle when the engine RPM is high enough for the pipe to be in tune.  On a 250, this RPM range is usually starts at about 6500 RPM and goes to the peak RPMs.   High performance two strokes should not be left at the same partial throttle position for more than a few seconds at a time!!!!!!!!!!!!   As the power level of the engine goes up due to improved porting and improved pipe technology the engines will tolerate less time at partial throttle with out experiencing run away detonation even with a well tuned carburetor, high octane fuel and correct ignition timing.