If the amount of SBD is worth more than a certain percentage of all Steem, then posting rewards will be only in Steem. I don't remember for sure what that percentage is, I thought it was 5%. Also the SBD peg to the Dollar loosens when that percentage is hit.

Given the huge drop in value of Steem, I guess that the 5% limit may have been reached. I haven't checked though.

Also note that Bitcoin has gained a lot of value, so the SBD to BTC rate doesn't really say all that much, SBD should be compared to USD.