Being denounced delivers a hit to your relations with that civilization (as does denouncing that civilization FYI). If two civilizations both denounce the same third party, they gain a bonus to their relations with each other. If you're that third party, they're going to unite in their mutual hatred of you sooner or later.
In such a case, look into finding a denouncement partner of your own against such a block, ideally a partner you also form a declaration of friendship with. Someone who shares a lengthy border with one of the two parties is likely to hate them, so they'd be the perfect choice to try to join up with.