SDB is a promise by the Steem Network that a user will get $x worth of Steem.
If people are selling off SDB than the $x worth of Steem in the future must not be that much(the relative value of the $ is dropping).
Another way to look at it, if Steem fails to rise in value and SDB also keeps dropping an infinite loop of ever increasing the amount of produced Steem by the network will disable the Steem Network. I'm assuming that is not going to happen(Dan and Ned have set money asside to stop such a devaluation).