Thanks no doubt to Counter-Strike: Global Offensive going free-to-play. With 609,382 VAC (Valve Anti-Cheat) Steam account bans for the month of December 2018, this represents the most bans issued in a single month. The figure also showcases Valve stepping up its enforcement for cheating in its games.
Here's a look of the graph, as found on SteamDB
Again, the huge spike in December comes alongside Counter-Strike: Global Offensive going free-to-play, which means that Valve must have been anticipating and planning to tackle cheaters head on. Even so, it's a remarkable number - and says a lot about the state of cheating as a whole when it comes to competitive games.