A significant portion of it is Yasuo's design since he is rather feast-or-famine at times.
Yasuo is a champion without ranged harass, ranged farming tools or waveclear or non-conditional escapes. As such, he has to be within melee-range of the minions to properly farm or fight, which exposes him to his opponent and can leave him overextended against the enemy jungler. If he falls behind, he tends to get screwed for the rest of laning phase against competent opponents.
If they aren't playing Yasuo, then a significant portion of that may be rendered moot since their new champion would have fallback patterns and lack some of the above weaknesses. Since they are no longer getting stomped or have alternative fallback patterns, they are less likely to be toxic since they can still be useful.
Also, while not the intent of doing so or something to encourage, it can have the side effect of helping to screen out toxic teammates during Champion Select. If the player decides to rage, troll-pick or ban another ally's pick intent over pettiness or revenge, it can serve as a good indicator that they are a toxic player.
Honestly, I would much prefer learning about that during Champion Select (where I have the option of dodging) than after I am stuck in-game with them.