It's hard to give a definative reason why, except to say its caused by excessive heat.

The reason for excessive heat can of course be down to many factors, the obvious one being excessive cylinder temperatures, a prime culprit being excessive boost on small turbine housings which creates very high exhaust back pressure.

Of course poor fueling, engine coolant problems, poor valve sealing, even head gasket proplems can and all do contribute, hence why theres no definate answer, except to say its knackered!