Really tempting fate posting this a couple of days before a derby. Bets on one of the two having a stinker tomorrow..?
More seriously though, think it's probably a couple of factors. Maybe new coaching, a different voice can make a difference. Suspect the return of Carroll has been huge as well. He's no Roberto Carlos (or even Rab McKinnon) but he's a lett back as opposed to a player that 'can play there'. Balance of the defence is far better...
I do feel it's worth pointing out though that for all the stick our defenders have been getting all season, there were periods where we weren't conceding huge numbers of goals. Losing 1 goal to Ross County, Dundee United and Hamilton should not have resulted in zero points from those games at the start of the season. If you can't score 1 goal against Accies at home or Ross County away, that's not the fault of your defence...