That is completely normal, and not a fault at all. It is simply a precipitation of tartrate crystals - potassium bitatrate - and only means that the winemaker hasn't been too heavy handed in the filtering or fining of the wine. The crystals won't affect the taste of the wine.
Sorry, I missed that you asked for the reason for their appearance in the wine. Tartaric acid is one of the acids in wine, and this acid will together with Potassium make a salt - potassium bitatrate. Despite the hideous sounding name, it is perfectly innocuous. It is not very soluble in a solution containing alcohol, and thus is likely to precipitate sooner or later, more so the lower the temperature. (Don't go changing your cellar temp over this.)
In general, winemakers will try to remove them through various processes like filtering, fining and cold stablization. This is only due to appearance, as indeed some people may think the wine is faulty if it shows crystals. (As indeed it is not.)
I assume that you work mostly with Australian wines, where, with a very crass generalization, the processes mentioned above have tended to be more or less legio. That could explain why you haven't bumped into these before. Fine german Rieslings, which tend to be lightly handled and especially high in tartaric acid, would probably be the wines with the most regular tartrate precipitation, although that is pure specualtion on my part.