As what is obvious by whats been going on in the last week valve made a change to vacnet that has done a couple of things,
AFAIK from what people has been pieced together by community members
A bunch of cheats and exploits got insta detected by vacnet or valve allowed it to make its own judgement leading to a ton of irregurlar behavior cooldowns.
Some of these cooldowns have turned into overwatch bans.

On the other hand there has been a bunch of false positives and the only reasonable reason people have thought of is people with horrendous internet or wack setups are sending inputs to the server in such a way that vacnet deems it as irregular behavior,
Similar to how bad keyboards can trigger the automated input kick,