My thought is she was only brought back during the second curse; she was off doing whatever those that weren’t caught up in the first curse were doing. I think she’s just emerging now because she’s got her sights on Emma, though, not Elsa. Emma’s powers are strong and everyone is realizing as time goes on just how much raw power Emma has inside her. It seems Elsa & the Ice Queen’s powers are rather limited – I mean, you can only do so much with ice walls and monsters – but Emma’s powers don’t seem particularly limited. Imaging already being quite powerful but getting your hands on even stronger and more varied power. She may have also just realized from Zelena’s silly kissy curse that Emma’s powers can be taken from her…so she’s got Elsa in town, the perfect scapegoat for anything the Ice Queen starts to do….she’s realized she can find the means to strip Emma of her magic…it’s perfect timing for whatever she’s got in store.