I think it may have to do with the fact that after HF20, the printing of SBD does not start to slow down when the market cap of SBD reaches 5% of the market cap of the virtual supply (STEEM+SP+SBD) but only does it at 9%. Simply put, more SBD got printed until the price of STEEM collapsed from 80-90 cents to 20-30 cents. The SBD haircut through conversion has to continue until its supply is again at a level where the SBD to 1 USD peg can hold. That will keep the number of circulating STEEM at a high level because in the conversion SBD gets burned and 1 USD worth of new STEEM gets minted.