1: Low hours in a lobby
This is VERY common in versus or expert campaigns because the lower hours you have, the more likely you're going to me kicked. Trust me on this because I do this as well, lobby leaders check each player's hours once they join and if they don't have the hours they're suppose to have, immediate kick. Now this seems harsh but you gotta look at this from the lobby leader's perspective because not a lot of people have the time nor patience to deal with noobs. The best way to deal with this is to put your profile on private or idle your hours because most people just check STEAM hours.
Note: The hours most people will accept is 500+ hours.

2: The lack of skill
Please guys, if you're not experienced in this game at all, don't play versus or expert campaign. Stay in single-player or in campaign with friends on easy or normal. If you play on versus, you're not going to have a pleasant time due to the insults and kicks by your team.

3: They want to make space for their friends to join
Group of friends may kick you in-game to invite a great player, also known as stacking. Don't really have to go into detail with this because there isn't much to say or do in these types of situations.

4: Your personality
I don't find this happening very much but if you type or talk to much = kick.
If you insult your teammates when they give you instructions = kick
If you're mute, meaning you don't type or talk enough = kick

Again, I don't see this happening very much (except for maybe the second one), but a way to avoid this kick is to see signs. If they're starting to get annoyed, or giving you signs on what you're doing wrong, either leave or change your ways.

5: You're being casual
If you just play the game on versus just to have fun by screwing everything up and laughing about it, expect a kick. Competitive modes such as versus do NOT have time for your nonsense. You either come in the game seriously or play a different mode. If you wanna be casual, make a friends-only versus lobby and invite some friends, just don't do it on pub.

6: ♥♥♥♥♥♥♥♥ unfair kicking
By The Lazygineer
"I remember playing L4D2 on my Xbox 360. It's just as fine as the Steam version but I have bad memories on it. I was playing a VS match on the game where there was this guy on my team with a buddy of his split-screening (Both of them on same team as me). They were horrible. Not saying about their skill but how they played. I was setting up an attack as a Charger and one of them came over beside me as a Hunter and started attacking me (First chapter of Parish btw). So another teammate and I tried votekicking, but it (obviously) didn't work. After a while that teammate left and a new one came on to replace him/her and then I got votekicked out."

If you see someone making ANOTHER post in the discussions like this, please link them this. I hope this helped the new players understand why they're being kicked and not to take it too personally.
Please, give me suggestions or feedback in the comments, have a good day everyone.

EDIT: This time I'll respond to everyone's comments if you leave any. This guide is trying to prevent even more discussions asking the same question that was answered over the years.

http://steamproxy.com/sharedfiles/filedetails/?id=502356107
http://steamproxy.com/sharedfiles/filedetails/?id=494668535
http://steamproxy.com/sharedfiles/filedetails/?id=508165123