An update. Just happened again.
I closed a Brave Browser window only for a page to open up promoting "Celo Ultragreen Week" trying to get me to enter into some giveaway. No chance I was clicking on anything related to anything like this.
Eager to figure out what the hell is going on. Malwarebytes isn't finding anything on my computer at all. Wondering if there's some sort of browser injection attack going on, or something.