"Bro Pages" associate this piece of software with bros, men. Such an association can imply that the creators and/or user of the pages will be "bros", men, as opposed to women. That could be "just a joke" if you didn't have a significant, visible people of guys who are obnoxious and immature enough to actually be hostile to women participating in programming.
When you have a hostile atmosphere, an overt (if ambiguous) statement that something is for someone else enhances, increases that hostility. A woman who is already facing hostility is certainly not "picking a fight" when she notices that naming something "bro pages" isn't a friendly gesture towards welcoming her. In fact, it is the opposite.
-- And that's not even touching the way "bro" has become synonymous lately with snicker, immature, sexist guys. Even "guy pages", "dude pages" or "pages of men" would be bad.
And yes, there is that hostile, defensive atmosphere on full display here.