It seems like Steemit Inc. is doing all the buying.
If I had to guess I think right now the budget or target they have for the price of a SBD is 1 to 1 with Steem... not sure if they will stop buying or change the target price to something else... no idea why they are doing it... in this article I analyze the situation a bit:
https://steemit.com/steem/@mexbit/80-of-steem-dollars-in-the-hand-of-steemit-inc-8-5-million-sbd