Various things in Dota 2 were updated including:

• Various bugs have been fixed in matchmaking.

• Weight has been adjusted to focus more on players that were offensive players.

• Matchmaking is now more sensitive to the number of games played for new players. Players who are detected as smurfs are treated on their skill.

• Behavior scoring system and recalibrated existing behaviour score values were improved.

• Exploiters, bots, account sellers, and in-game feeding were all detection improved. Matchmaking bans apply.

Source: http://www.dota2.com/news/updates/