Allegedly, on December 31, updated anti-cheat methods were rolled out to various Counter-Strike: Global Offensive servers. Namely, it appears as though the servers that Valve run themselves were updated while other dedicated servers were not yet updated with this "mysterious" update.

Those of the CS:GO section on Reddit noticed that a larger than normal number of cheaters were bitching that their undetectable cheats had gotten them banned from VAC secured servers. If this is indeed the case, Valve made no announcement of this, perhaps to catch these cheaters off guard.

In addition to this apparent upturn in these private hacks being banned, Valve is also allegedly starting to dish out one-year bans from the game for those that purposely crash servers through exploits.

Right now, these are just assumptions from those in the community that keep an eye on things like this. There has been no word from Valve on this matter.