According to reports across multiple websites, a new update for VAC that detects new cheating programs was released on New Year's Eve.
It appears that Valve is conducting efforts to make CS:GO a more enjoyable experience by getting rid of cheaters and also players who try to crash game servers on purpose.
Over the last 24 hours several players have reported that more cheating tools are now being detected by VAC, resulting in a huge number of bans.
It also seems that all Steam users that are caught deliberately trying to crash Valve's servers will be handed one-year bans and will become 'untrusted players'.
Valve has so far decided to stay silent on the matter, probably to prevent the developers of cheating tools to know exactly what has changed in VAC or what they can do to counter this update.