Why are these champs often banned?

They are all high-ceiling champs that can be abused by mechanically gifted players that have huge outplay potential.

Because they are popular champs who are designed to make players stare at a grey screen instead of playing the game, which people don't like because they would rather be playing the game.

They're annoying regardless of how weak or strong they are.

You know someone is banned a lot when they're listed twice. :P

But anyway, from the ones you listed, it's got a lot to do with frustration. Someone who's good on any of those champs is incredibly frustrating to play against, and the fact that they all snowball faster than an avalanche in the Himalayas means that if someone feeds them, there's not a whole lot you can do to stop them.

Individually, Lee's got a lot of tools in his kit, including insane mobility and a lot of safety, plus a lot of damage naturally built into his kit. He's very forgiving for mistakes, which makes it hard to punish him. Camille is similar, in that she offers mobility and safety, not to mention her lockdown ult, but I'd prefer fighting her over Lee any day of the week. I actually think Camille is a little on the weak side, but saying that is likely to get me crucified, plus I haven't seen her a lot lately anyway. Zed is very difficult to pin down as well, thanks to (again) mobility and a very tricky kit. LeBlanc is similarly annoying for the same reasons, and both can burst you down pretty quickly with one or two abilities. Kha and Rengar are both similar to each other, since they both perform the same job - turn invisible, jump on you, burst and you die. Rengar is a bit easier now since his ult has an indicator, but Kha will melt you if you're caught out. Riven, Fiora and Yasuo are all annoying thanks to the steroids build into their kits and how easy it is for them to outplay you while not giving you a lot to outplay -them-. Riven has 4 dashes, a shield/stun, and a knock back, Yasuo has his passive shield, dash and windwall to protect himself, and Fiora has her own dash and parry.

All of these champions are not champions you just pick up and win with, but when someone does devote time to mastering them, they dominate, and I personally think it's a design mistake to consider "a steep mastery curve" as a weakness, because if that's the only thing holding you back, then it means that once you get good enough, there are no longer any weaknesses, and that is not healthy champion design. Some of these champions are better than others in terms of counterplay, but they're still incredibly unfun to play against when someone knows what they're doing.

Mostly because they A: kill people. B: Snowball really hard. C: difficult to deal with when ahead.

because they're good.

Look at their kit movability and burst. People hate these types of champs

My guess is - Lee Sin is a versatile fighter with strong gank capabilities, Camille is a strong skirmisher with both strong offense and defense and mobility, Zed is a situationally strong anti-ADC champion, Kha'Zix is incredibly hard to keep track of with his evolved R, and also very quick to lay on hurt with his evolved Q.

Rengar is suitably snowbally, gaining sizable increases to damage simply for killing the enemy team at least once per champion, Riven has a fairly strong damage output with reliable defensive tools, Fiora is a strong duelist with good survivability and the ability to turn teamfights in her own team's favor, and Yasuo is another strong duelist with a reliable ease of snowballing with crit damage, as well as some reliable CC.

But, I can't say for sure what's the reason behind each ban - it could be exactly what I said, or it could be that people know those champions are counters to their own pick, etc.

players are always ass when theyre on your team and hard carry when theyre on the enemy team. it's known as the yasuo effect, and also applies to