This is one of those cases where I think gendered expectations do come into play in the sense that women are supposed to be agreeable and compliant, looking out for any supposed underdog. A lot of SciFi/Fantasy readers imagine themselves as outcasts of one sort or another, and I think they saw Rowling as someone who understood them and wrote to them. Because she has stood up for her principles -- which they consider the wrong ones -- they can't forgive her, especially when we are in such a crazy cultural moment.

In contrast, Gervais has always been snarky and has a very different fan base.

As to how it got this bad, most cultural analysts are in agreement: Social media in general and Twitter in particular. It is having a corrosive effect on our ability to function as a society where people with different points of view can live together without constantly being at war. People now see this constant bickering as normal.