It is not the job of witnesses to maintain the SBD peg.
It is part of the job of witnesses to publish the actual STEEM exchange rate (aka price feed). It is also part of their job to publish an interest rate for SBD as they see fit.

IMO the current disparity of SBD/USD is due to two main reasons:

• the downtrend of the STEEM price means that SBD->STEEM conversions take place at an exchange rate that is too high, due to the 7 day median
• the market is not sufficiently informed about SBD, in particular I think that many market participants know neither about the pegging mechanism nor about the interest rate

Both points are out of scope for witnesses.