Thanks Yuu! I was just wondering the same i.e. whether this is happening just to us or in other fandoms as well. I changed my password earlier, but got another try just now.
I suppose someone just finds it exhilarating to think how they are making everyone puzzled. Other than that, it seems kinda moot.
It's definitely happening in multiple fandoms. Don't worry, it's not just us. If you already changed your password, it's unlikely you need to do it again.
I looked through discussion on Dreamwidth related to this issue, and the current suspect is a malfunctioning spambot. Anyway, Ao3's PAC is definitely aware of the issue but will probably take a while to make an announcement about it.
I locked it. If I unlock it in a few days, will the bot desist?
Short answer: I don't know.
Long answer: For works already posted, I'm unsure if that will work, but it won't hurt to try. For new works, if you post it locked, leave it for a day or so, and then unlock it, that might prevent the bot from hitting it. At least for a while. (But this is only anecdata - one of my mates was getting hit by it repeatedly whenever she posted something new, and this helped.)
It's possible there is more than one bot - the one that does this is a type of crawler that goes in and pushes every button on the page that it can find.