SBD are not "pegged" to a dollar in a traditional sense. They are redeemable for one dollars worth of steem. You can't break that kind of peg, if steem goes down in value you just get more steem for each SBD and if it goes up you get less. Obviously holding SBD has more risk than just holding a dollar because there is a chance that no one will want the steem at any price. that is why the SBD pay a yield for holding them. SBDs are much more like a convertible bond then a dollar pegged crypto.