I ended up staying on after Elth left last night (meant to stop, but was enjoying myself too much). I can't comment on the other Maps, but on the Sanctum Borderlands, it wasn't so much a surge in our performance that reclaimed the map, but more a stubborn refusal to give up that broke Darkhaven's momentum.
Darkhaven looked unstoppable, and eventually had everything except Garrison Keep and the two towers either side. We couldn't seem to stop them taking the towers, so we simply kept retaking the tower on one side of Garrison when they went for the tower on the other side (we had a couple of Trebs up on either side of Garrison to break down the walls of these towers quickly). Eventually Darkhaven went for broke and tried one final push on Garrison, using 3 Golems (at least 3 that rendered for me) to break down the Water Gates in record time. After an epic fight in the Lords Room we somehow prevailed and Darkhaven's spirit seemed to be broken. Within an hour we had the rest of the map back with virtually no opposition (and it looked like the other maps were starting to do the same, when I logged the centre map was also ours again bar 3 objectives)