Probably either tartaric acid crystals (which is a common and normal sediment for red wines (but that is a lot)) or yeast sediment stained by the wine.
Organic producers don't add preservatives so maybe there was a little in bottle fermentation over time. Or they didn't use clarifying agents, or didn't microfilter and some existing material in the wine can come out of suspension over time.
But that amount is still lot in either case. If it is yeast sediment it is also possible they were taking from near the bottom of the barrel and during bottling.