It is called 'flocculation' (stop laughing). There are two types, the first is caused by cold temperatures and is reversable. The industry has dealt with this by chill filtering.
The second is known as 'irreversible floc' This shows itself as very small hair like crystals of calcium oxalate, which slowly form and settle in the whisky when natural low mg/l concentrations of oxalic acid in the whisky react with similarrly low concentrations of calcium ions. Irreversable floc formation is eliminated by ensuring that final calcium concentrations are kept to a minimum by demineralizing the water in the final reduction.
Now you know.
The above was stolen, almost word for word, from Whisky, Technology, Production and Marketing by Inge russell.