Thinking more about it, along with pulling back on the spine instead of just pulling downward, in this situation, I may have ended up in a position similar to your avatar photo where I really wasn't taking as much weight off the rope below the BDB making it harder for the bollards to bend the rope to gain a bite. Also, with the BDB in a position similar to the climber's hands in your photo, the angle in the rope may "pinch" the arms making it harder for them to separate. Add each small piece to the scenario and the sum must have been just enough to create the situation....