turbo charger replacement question.

[RCtime]

I contacted the Cat. hot line with a question and while there asked them for previouse maintenance history.
They said the Turbo charger had been replaced plus all gaskets and such.
They said it was smoking and oil from the exhaust was the complaint from previous owner.diagnosis, bad turbo.
What would cause this? Would this be caused by not cooling the turbo prior to shutdown? It sure has run fine for the last 2 years.