There is some tribalism I think but mostly people just want to like every movie and wish the DC movies were more to their tastes. And also things just snowball on the internet; when people like something it becomes the greatest thing ever, and some people go too far. And when something becomes a negative meme, it’s hard to turn that around. And lots of people go too far (such as making fun of Affleck’s alcoholism or Snyder’s tragedy).
But then the content machine and “critics” get out ahead of the clicks and provide people want they want to hear, really no different than political content. Mother Jones won’t run an article about how Trump’s policies have led to job growth and geek sites would never run a “10 things we love about Dawn of Justice” article. I guess that could be seen as tribalism but it’s also just the internet economy.
The whole thing with the DC movies has been bizarre. I don’t think people in a broad sense disliked Man of Steel, but they really hated Dawn of Justice. But then they loved Wonder Woman even though it’s really not any better.
If they just did Man of Steel, and then Wonder Woman, and then had those two characters join Bale’s Batman for a Trinity movie, then it would have been DC’s Avengers and lots of people would have loved it.