Bans served to accounts found abusing matchmaking.
DOTA 2 ban hammer

On Tuesday, February 11, the official DOTA 2 Twitter account revealed that Valve banned over 40,000 accounts from the popular MOBA. The studio says that the bans were handed out to accounts that were "found abusing matchmaking."

Details on what this means were not shared publicly. However, it could be inferred by what was meant by "abusing matchmaking." This could be anything from buying accounts that are of a high rank, smurfing, or boosting. Maybe there was even a little bit of botting.

Smurfing being when players of a high skill level play on a secondary account in order to be matchmade against lower ranked players. Boosting would be if you kind of rode on the coattails of players that are of a much higher skill level than you. These highly skilled players would carry their lesser skilled friends to high rankings that they shouldn't actually be at based on their skill level.

There may very well be other ways in which matchmaking was abused, but that's for Valve to know and us to merely speculate on.