Upset the balance. Sidhe trying to change the balance between Summer and Winter has been a recurring theme - three times so far in the series, by my count, two of them initiated by Summer. (Unless I misremember, I mean, but I think that's the score).

It's the way Sidhe are playing around with the White Council, including Summer. And the way Titania hasn't been friendly with Harry or the WC, even before Aurora's end. IDK. White Council is for maintaining the Status quo - and Titania obviously does not have too much liking for it.