There is always a flip side to every coin. My personal guess is they are selling shares that they will buy back later at a premium from the purchaser as a short term loan to fund new large orders till they get the accounts revolving. I think the earlier buybacks support this theory, but it is a theory.
They do not want the less than honest brokers to know the amount so that they do not flood the market with NS shares not cleared through the DTCC.