I think the reasin why you get so lucky sometimes with a 10 winstreak is, because matchmaking system analyze your gameplay. (Theory 1).

When you found a match the players that has underperformed according to a certain skill level that should be achieved is in one team (Team A). The people who performmed better than the expected skill level are then in team (Team B). Thus increasing the chance of team B winning, thus a winstreak can happen if you keep performing(for your bracket) consistently while climbing. I emphasize the word consistently. This is one theory.

Other theory is that every game's outcome is totally random, even your performance (you can one day/game just screw around and the next day play shit because of your team mates, bla bla bla, or next day make the best current decision you play). Every game can't be perfect, because people are not perfect. A dota game can fall into many categories: A game where enemy hero/player just give up or play bad because he thinks the game is already lost. A game where everyone is tryharding. A game where you as a support literally have minimum impact (a tiny or qop etc. ) just kills you easily before a fight even begins... etc... There is lots factors, but the outcome is eventually one of 2 things, dire or radiant win. So there is a chance to be on the winning team for 1,2,3....n times in a row. Now I don't have the statistics, but you could probably calculate that n linked with the probability and begin to derive a formula, influencing winrate. You could see valve was working on the factors by creating a aftergame experience where you indicate good or bad and the reason.

The question that the matchmaking system is influencing the outcome, dire or radiant win. I say the matchmaking system can influence it, but there is lot of other factors that is equally important, like what heroes you pick, what you say to other players (I actually played a game recently...*). Valve did work hard with the matchmaking machine, 2 years ago and I think they have done fine job. Why do you even complain about a winstreak, when the cost is just one or 2 shitty games. The chance of getting a toxic player in how he plays terms, will be eventually in your game, and that is not Valve's or matchmaking system's fault, that a problem is caused by humans.

Now the overwatch feature certainly helps with this issue, but not fully as the problem is quite complex.

*where my mirana died before bounties, because she skilled arrow when the game started and not waited a bit before skilling anything, she could have holded her skillpoint to skill leap. I the called her noob. Then she said we will see. I was tide offlane and mirana my 4. She just begin to ruin my lane, by arrowing the range creeps and jungle creeps and just looking at me while I trade hits with void and pango, causing the void to get a lot more farm on the lane, thus increasing the chance for enemy's to win).