The Steam Summer Sale of 2017 is over, but something just as, if not more, extraordinary happened the very next day. On July 6, Valve’s anti-cheat efforts dropped some digital judgement upon well over 40,000 Steam accounts. This is the largest mass-banning from the service in its entire history, landing at over triple the previous highest number.
This figure isn’t entirely surprising, however. Steam event sales are a source of mass account bans, as these events are usually seen as an opportunity for previously banned users to try to recoup their losses with a new account. Making a new account after being banned is fairly trivial, but making a ton of purchases paints a big target on your back, apparently.
It's also notable that over 5,000 in-game bans were dished out as well on the same day. According to a service that tracks bans and their effects on in-game economies, this caused a pretty huge ripple in the CS:GO weapon skins market, to the tune of over $9,000.
After July 6, the number of bans settled back down to normal, so this was definitely a conscious effort to combat specific activities.
Source: PC Gamer