heat seizure grabs the piston and kills the engine when you slow down.. it's gonna be stuck for a while until the thing cools off a little. Seizure could be due to lack of piston-to-cylinder wall clearance.. new rings that weren't gapped and are now too tight.. lack of oil in the fuel mix.. glazed cylinder wall.. But these (and more) will show obvious evidence in the cylinder and/or piston's condition..
And then there's too high compression.. too advanced timing.. leaks in a (performance) exhaust system .. hard running in hot, dry weather combined with borderline-lean fuel:air mix.. blown head gasket.. intake vacuum leaks.. high exhaust backpressure.. and about a million other things that can cause very high combustion chamber heat.
hard to guess what's up without knowing lots more about the bike and it's history.