Valve has banned over 40,000 Steam accounts following its annual Steam Summer Sale. It shows a relatively steady line of around 3,500 bans per day (give or take a thousand on any given day) up until June 6, the day after Steam's Summer Sale came to a conclusion. Easily topping the previous record of 15,227 (according to Steam Database) from October 2016.
An additional 5,000 users received game bans on 6 July, which ban an account from playing a certain game online. Cheaters have been making their moves during the summer in Dota 2, and even more so in Counter-Strike: Global Offensive, because of the cheap workarounds brought about by these sales. It's no great trick for a banned Steam user to set up a new account, but the cost of re-purchasing lost games can add up pretty quickly. Why did Valve ban so many accounts in a single day?
On July 7, the day after the first hit of the ban wave, 954 VAC bans were handed out, while another 735 were delivered on July 9.
When it comes to Valve, the Steam bans often happen as a result of cheating on some of the most popular games on the platform. All of these accounts end up banned due to it picking up cheating, and that is a lot of accounts per day on Steam that end up banned. When a player is banned by the VAC, they are no longer allowed to join Valve servers, which make their skins and other digital items useless.
As noted by Dot Esports, the timing nearly certainly isn't coincidental. That in part is why the Valve Anti-Cheat System was put into place.