It's a tiny handful of people who are fixated who are driving it, imo.
There was some investigation online that found that just a small number of people were running hundreds of separate Twitter accounts dedicated to Meghan hate.
On Buzzfeed, whenever there's an article about Meghan, you'll see immediately see a whole bunch of comments all posted within a timeframe of a few minutes slagging her off and calling her an abusive narcissist and repeating crazy conspiracy theories about it, and when you click on their profiles they all registered that day and have no other comments. It's really obvious it's just one extremely obsessed person who just mass-creates new accounts, uses them to make a single comment, then ditches them. As part of a plan to make it look like Meghan is widely hated.
There's one poster on here who goes on other forums (the kind of forums where Meghan is routinely called the N word and worse) bragging that they've been banned from Mumsnet more than 30 times and that they keep making new email accounts so they can create new Mumsnet accounts to keep posting Meghan hate. The sheer scale of hate threads here is crazy and they are often started by posters who never post outside of Meghan threads. During the engagement there were like 10 new Meghan bash threads per day, that's not normal.
Obviously a big part of it is racism. But a huge part is the rise of "parasocial" Internet-celebrity relationships. When Benedict Cumberbatch got married, a small group of extremely obsessed fans got fixated with hating his wife because she took him away from them, and for eight years now they've essentially made it their full-time job to spread hate and deranged conspiracy theories about her online. It's literally like half a dozen people with hundreds of fake accounts who each spend hours every day posting that she's a bitch, a prostitute, a fake wife for PR, their kids are lifelike dolls, controlling, abusive, all the same things they say about Meghan. Except a couple of them really went off the deep end (clearly mental illness is a factor) and invented this whole paranoid conspiracy theory where Cumby's wife is the head of a Satanic cult and a major human trafficker. I guess they got sucked into Q-Anon stuff too and the Q-Anon theories are colouring their obsession with the poor wife.
Internet communities really groom people into these cult-like hate groups, and give unhappy frustrated people who feel voiceless an outlet for their misery and anger at the world, and make them feel important and like they know things about the world that no one else knows. It gives them a purpose in life: expose this Bad Person/Thing! These groups also make people feel empowered because it's proactive - they "research" their target online, and if they find inconsistencies (like a celeb says slightly different things in different interviews, or a celeb's pregnancy bump looks slightly different at different times), rather than brush it off as human beings being fallible and changeable they'll take it as evidence and vindication that they're right and should keep going, get validation from others in their group, and basically act like Woodward and Bernstein.
Doesn't matter if it's Q-Anon, Sophie Hunter hate, Meghan hate, Larrie truthers, white supremacy groups, flat earth, anti-vax, it's all the same psychology.