Is there any reason to think it isn’t rooted in the difference in political alignment?

Yes, there is. Lemmygrad is politically aligned with HexBear, but receives far fewer complaints from users of other instances. Why?

IMO, it’s because Lemmygrad users typically mind their own business and keep to themselves within the confines of their own instance. Hexbear users on the other hand …

Although I agree that the majority of new Lemmy users are probably not used to political challenges coming from leftfield as opposed to from the right, it seems to me that the core reason why Hexbear users generally get under people’s skin to such a degree is their overall behavior, not their political stances per se.