unburnt fuel.

Ive smoked out a few people when theyve been unwise enough to stand behind my landrover on start up

As has been mentioned, its due to a number of contributing factors

usual service items like airfilter, fuel filter

Worn items like fuel pump, injectors.

Also things like a cracked or small leak on a turbo vacum hose, even a pin hole size can have an effect.

I wouldnt worry too much, unless is very heavy smoke. Like those old transits you see doing a james bond smoke screen down the dual carrigeway.

If its just on start up and under heavy load dont worry. Even brand new diesels will puff away if you floor them.