I can only suggest to try and find some blog posts or youtube videos about the update, and what kind of websites were hit the most to see what you can do to recover. As for "Why" nobody really knows.
We all know it hit affiliate sites the most, but many informational sites are hit as well. For example, my main site was hit and now I earn only a third from that site compared to before the update. So you're not alone in this. I also am not sure what caused it, but leaning towards Google punishing multi-niche sites. If you're a multi-niche, all I can say is increase your topical authority by writing more. I've also added an author box on posts on my main site, but I'm not sure if that will do anything as I'm still losing snippets like crazy.