The peg had failed in 2016 but for a long time before the August 2017 whitepaper, the peg had held. So I am disinclined to believe that was the reason for the name change but I can see how the name change modifies expectations:

STEEM Backed Dollar - a cryptocurrency whose US Dollar value is backed by STEEM.

Steem Blockchain Dollar - just another cryptocurrency whose value is determined by market forces. If this is the case then why not just discontinue SBD by converting all of them for STEEM and simplifying the dynamics of the platform for easier adoption and growth?