There is a tonne of possible answers to that question to be honest. Usually poor servicing and oil changes are most commonly at fault:
Poor quality oil, oil not changed regularly can thin out and not offer sufficent lubrication/cooling to the bearing of the turbo.
Contamination in the oil due to build up of sludge etc (arising from not be changed enough) can block up oil feed or return pipes causing starvation.
There is also the possibility of the turbo sucking something in like a big piece of dirt or in one case I've seen a piece of a paper air filter that hadn't been changed in God knows how long.
If the turbo has failed you need to clean the entire system out before you put a new turbo on.
This means the air filter, induction pipes, intercooler and pipework. Parafin is really good for this.
The oil feed and retun pipes should be replaced if possible or at least cleaned out thoroughly.
An engine flush should be ran through. Sump removed and cleaned out. New oil filter.
You get the idea!
Damien
