I'm inclined to agree with your explanation. A couple of extra points: apparently a major Korean exchange adopted Steem and SBD recently (learned this from @netuoso), and also @alexpmorris made a point about how traders shorting cryptocurrencies can cause them to moon. Basically when SBD initially started to surge in the way you described, some people with faith in the peg mechanism would have borrowed SBD (usually with interest) to short it. The peg mechanism is actually very weak though, and the longer it went without going down, the more they owed in SBD. Thus they are forced to buy at a higher price, which propels the price upwards again in a short squeeze, and other technical traders may jump on board seeing the "technical move" at the same time.
I think all these things came together in a perfect storm after the Tether hack.
In the long run the weak downwards peg should still bring the price down. It's just in no way reliable in the short term. Hopefully we can bring in tools to improve the downward aspect of the peg.
Good points! Do you know if Bittrex lets you short SBD?
I don't think so, I believe it can only be done ad hoc for now.