Yeah, just checked on the itch client status and, while there are third-party clients mentioned in the readme of their source code, neither they nor the main client had updates that remotely coincided with the uptick in downloads. Even if they did, they're Android and iOS clients, so it'd still be baffling.
Given that things returned to normal, my own (weak / unverifiable) hypotheses are either "A person trying to download the updates had connection issues" or "Someone with a lot of reach reminded their friends, who had already bought CS:GT, about the updates a while back."
Thanks for bearing with me, I just like mysteries like this, hahah.