I think it was a reaction to the mainstream MEDIA narrative that everything was his fault and she was nothing but an innocent victim. Depp got EQUALLY as much hate and cancellation when her accusations originally came out, if not more, as Amber Heard got later on when the story became more complicated. I truly believe that people would have been a lot more nuanced about the whole story (that they were abusive and victimised in different ways) if the original reaction to Amber Heard's accusation wasn't "believe every single thing she says, blacklist Johnny Depp from every opportunity, destroy his career and publicly talk about him as if the accusations Heard made about his violence were all 100% true". Except I remember what it was like and absolutely all my friends were on her side, and believed he was the scum of the Earth, and that anything short of absolute blacklisting was a sign of abuse apologism. So when it came out that several of her worst allegations had very little evidence behind them (the one that he allegedly bashed her head against a wall, used broken glass to cut her etc) - plus then evidence of her mocking him and admitting she hit him came out - it's natural that people started to hate her.

Do I believe that both Depp and Heard were toxic and abusive? Yes. Do I believe the worst of Heard's allegations? No. Do I believe the worst of Depp's allegations? No. (It's why both were found guilty of defamation). Do I believe that the cancellation of Depp in 2018 was premature? Yes. Do I believe that Depp and Heard got around an equal amount of hate except at different times? Yes. Do I believe a large majority of the hate against Heard was sexist? No. Some was but I think majority was a backlash and overcorrection to how fervently the media and social media had been on her side before evidence came out that she had lied about major things and also been abusive herself.