Weird. The results are in the exact same order for me when I clicked both of your links. The only difference is that the bootstrapshuffle.com result is below (or rather, on the second page) the stackoverflow result on Bing. Considering the former is specific to Bootstrap, it's less relevant than the stackoverflow result.
Anyways, DDG does mainly use Bing, but they also do their own modifications to the rankings, which is sometimes for better and sometimes for worse.
I'm a couple of days late to this, but no, I cannot provide a reference (anymore). DDG used to have a page that described in pretty great detail how the results were sourced and ranked, but it seems the current page is buried pretty deep and has less information specific to ranking. IIRC, the Bing results (and previously Yahoo/Verizon Media) rankings were augmented by their own crawler (DDBot) and aggregated usage of their search. I believe they also pull the results that their "instant answers" are derived from higher up based on the search. If, for example, you did a technical search and their "instant answer" was from a stackoverflow post, that particular post would be pulled higher up the list (but not necessarily to the top) even if it was lower on Bing.
Anyways, DDG does mainly use Bing, but they also do their own modifications to the rankings, which is sometimes for better and sometimes for worse.