They are not against Israel. They are against the policies of it's government. It's like saying that those didn't vote for Trump are anti-America. Really. Maybe they want to save the US from itself

Here's the explanation.
It does seem strange, but is a well known, and predictable phenomenon. It can happen in any group when demonized constantly. Please see
http://www.westernspring.co.uk/the-mental-disorder-of-white-self-hatred/

"Essentially, what happens is that when a specific group of people is subjected to constant hatred, demonization and abuse,...

These despairing individuals, ... will actually take the side of those harming them against others in their own group."