How is that passive voice?
I don't know... how can a "generational divide" possibly "flare?" A "generational divide" is an abstraction, an intellectual construct.
The author of the piece substituted an inanimate concept - which can't "flare," or do anything else - for one or both of a pair of human beings who can. McCain took the wrong side here, as elderly Republicans usually do, attacking one of his fellow party members from behind.
By making it sound like the "generational argument" is the issue instead of John McCain being the issue, the author took the passive voice, as I understand it.