I always thought of it as Starbuck reacting to the destruction of the Colonies by wanting to desperately hold on to what he already had with Athena. Probably more a knee jerk reaction, but still based on what he obviously felt for her.
And, conversely, and likely in self-defense, Athena pushed him away, afraid to take the chance that she would lose him too. It wasn't that she was less caring than Cassiopeia, or didn't love SB. She just couldn't or didn't want to allow herself to care about any one person, at least at that point. Her reaction to the tragedy.
After all, if that scene in the locker room between Starbuck and Athena had turned out differently, the subsequent scenes with Cassie might have never happened.
At least until after Starbuck and Athena broke up . . . :twisted:
I also think that the loss of Zac, while she watched, plus knowing that SB traded patrols with him, may have left her feeling more confused about their relationship than might have otherwise obtained.