I don't know all the reasons, but the main things I've been hearing about are the issues that people have with Ilvermorny(it is a copy/paste of Hogwarts, prioritizes White wizards over Native wizards, gets historical facts wrong), that the Wizarding World has a few unaddressed problems(child endangerment, biases against Slytherins and Hufflepuffs), the new Pottermore with its redesigned test and lengthy articles, and the FBAWTFT franchise that just seems to be milking the Harry Potter franchise. I don't necessarily agree with all these, but they're just what I've heard around the Internet.
tl;dr: People don't like changes to the canon and franchise and are looking at the books with adult eyes and finding fault.
I think it's not just that, but she's hugely liberal and is Tweeting a lot about Trump, and she's run out of fucks to give about offending people who tell her to "get out of American politics, you're British"
Well, it's the 1920's, so of course they would prioritize whites over natives, and she really needs to shut up about American politics, especially when she starts getting stuff wrong and then just doubles down instead of apologizing.
Nearly half of America voted for someone KNOWN for their "saltiness" on twitter. But hey, if he's on your "team" he gets a pass for exhibiting traits you find distasteful, right? ;)
tl;dr: People don't like changes to the canon and franchise and are looking at the books with adult eyes and finding fault.
Remind you of anyone we know? ;)
2. How dare you insult our Queen?
That doesn't mean she's got to be so salty on tweeter.