The really short answer seems to be: actix was a Rust web framework. It did some things internally that some people didn't like. The Internet echo chamber picked that up and amplified it to a volume the maintainer couldn't justify working through. The Internet echo chamber is now going to pick this up and amplify it to a volume that probably isn't justified either.

I think this is another story about the destructive power of the internet echo chamber, where the asymmetry of one person vs. a mob of people who are in reality probably not all that engaged with the problem but nevertheless have enough engagement to send a nastygram or two creates a distributed denial of decorum attack that no single human being should be expected to deal with "nicely". Our ancient instinctive tribal signals of whether or not you are approved by the tribe, tuned for tribes of 150 people or so, receive a message that 15,000% of the tribe thinks you are a bad person, and our natural human response to that is a lot of stress at best (our ancient instincts tell us that eviction from the tribe is a bad thing, even though in practical terms eviction from this particular tribe won't be much of a problem at all in the modern world).

(I phrase it this way because I think this is, well, not strictly speaking independent of the question of whether or not the maintainer was guilty or innocent of any particular thing, but because the story is the wild disproportionality of the response you can get on the Internet regardless.)