I agree with @timcliff that at this point the action in SBD has little to do with STEEM or Steem users. It trades more or less independently on the Poloniex exchange (and others, but Poloniex is the largest) and in volumes that are far greater than what Steem users are responsible for.

What drives the SBD price is the value that is attached to it by speculators as well as overall demand in the cryptocurrency economy for stable tokens (especially given the problems that have surfaced with the market leader Tether). We've seen the same sort of sudden and large surge of trading into BitUSD (bitshares dollar token), except that in that case the peg was held much better because Bitshares has the ability to create new BitUSD tokens as needed to satisfy demand (by shorting).