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It over heated - at least the piston did - I see it's a Wiseco M series piston - you say it's a new top end - what by your definition constitutes a 'new top end'?

 

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How did you break it in and what kind of oil did you use for break in?

 

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Did you let it idle longer than a minute after riding for 30 minutes?

What was the piston to cylinder clearance set at prior to assembly?

Type of oil is not important to anything other than the clutch - if the oil does not have the JASO rating, it can cause the clutch to slip and fail.

 

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Wiseco pistons are forged so they take longer to warm up and expand more when heated so more than standard clearance is required when at room temperature - whoever sold you the kit should have set the proper clearance, but they are not machinists and you are not either, but the proper clearance is the responsibility of the mechanic assembling the engine.

Regardless - the problem was the result of overheating and the piston got too big for the cylinder.