It is actually somewhat peculiar that SBD demand would increase when STEEM price increases. There is no real mechanism by which this should occur, other than reduced risk due to the higher STEEM price reducing the debt ratio, but (for the magnitude of price increases we've seen) that is reasonably negligible. It may simply be self-fulfilling in the market (people expect it to happen, so they trade on that expectation, and it does happen). Likewise there may be some bots that in a similar fashion trade on observed correlations that have become self-sustaining. In time I expect this effect to reduce.