Update: The report originally indicated that all 11,000 bans were handed out to CS:GO cheaters. However, vac-ban.com indicates that fewer than 4000 CS:GO players were banned that day, while the balance, according to this thread, went to Dota 2 players. SteamDB doesn't break down VAC ban numbers by game so it's impossible to verify without confirmation from Valve, which I would guess will not be forthcoming. Even so, it seems very likely that this was a bad day for CS:GO and Dota 2 cheaters.
Today was not a good day to be a cheater. According to a story by Kotaku, more than 11,000 people have been banned by the Valve Anti-Cheat system for breaking the rules, one of the largest spikes of VAC activity this year.
Valve Anti-Cheat is continually banning players, but in this case it appears that the system has become able to detect previously untouchable cheats. The Kotaku report includes an image of “banned dickheads sobbing into the empty space where their knife collections once rested” (that quote is just too good not to use). The Steam inventories of banned players are essentially frozen: they cannot trade or sell items from their inventory for that game.
There's also an indirect acknowledgment from a “large cheat provider” in this CSGO subreddit thread that his software isn't currently performing as it should. The thread also contains messages about people with previously “good” cheats—ie, which VAC could not detect—who have suddenly found themselves locked out.
The tricky bit about this sort of thing is that these ban waves very rarely come with explainers that break down how and why a particular round of bans was implemented (although I guess the 'why' part of it is fairly self-evident), or even to confirm that something out of the ordinary has happened at all. But the war against cheating in online games is an ongoing one, and so it only makes sense that, as systems improve, there will be these sudden upticks in activity as Valve 'cracks a code' somewhere and trips people up. Of course, the opposite holds true, too: players determined to cheat will come up with new ways to do so, and around and around it goes.
I've emailed Valve for more information about the sudden uptick in bans, and I'll update this post if and when I receive a reply.