It does it in the early hours quite often, and noisily too; I may try it manually just to see if it makes any difference, though NTFS isn't the greatest (bit sad if it is actually based on ODS-2 as that was always pretty good). I do have some
SSD space but it's definitely at a premium; I suppose it may be worth redirecting Steam's downloads directory onto it though.
Mostly it's just because Steam's unnecessary pre-allocation is a terrible design choice, and evidently very poorly implemented. People have been complaining about it for years but it falls on deaf ears. :|