But why now
Why not? There isn't any specific reason to "why now"; There was way too much plagiarism content, so someone decided to create a bot... and that's it.
and why so many?
Because it is a bot after all. It is probably checking old posts, and possible every new post created. Checking that many posts and reporting them is inhuman so a bot can handle it pretty well.
Did they develop an automatic tool to check members' posts en masse, then moderators check suspicious posts manually and give out bans as required? Are there any false reports? In short, how does it work?
A bot reports, the mods check, if it's plagiarism = ban. Easy.