SBD (and STEEM) was recently announced as being added to upbit (a Korean exchange). It created a lot of hype, and since SBD has a relatively small supply, the demand seems to have caused a pump. Since then there has just been a lot of speculation trading based on the high volume it created.

Normally (when SBD is around $1 USD), there is a large amount of new STEEM that gets generated each day via SBD conversions. Now that SBD is trading at a premium, most likely a lot less will be converted to STEEM, so this can have an indirect positive effect on the price of STEEM too.