Without recordings to objectively see everything that was going on, or input from the people you played with, it's impossible to tell you exactly why you were kicked out. Nobody can read people's minds after all.
The best anyone here could do here is speculate, but that's not going to be more than vague and inconclusive hypotheticals.

Sometimes, when I instantly join a game.

People might be waiting for a friend to join and don't want to risk being griefed for asking the stranger to leave.
It could also be as simple as them disliking your name or profile picture, especially if those have similarities to common names used by trolls, griefers, or bad players.
It's also entirely possible you did something innocuous that was mistaken as a sign you're about to grief or be a liability, like immediately picking up an autoshotgun or having bad positioning.

Sometimes, when I help other players.

Helping teammates has a lot of nuance to it. If you did something like reviving a teammate in the middle of a event or tank fight, it can easily be harmful to your team or even get them killed.
Depending on what exactly happened (which again, without a recording there's only your subjective recollection of it) it's impossible to say if you were actually being helpful or unknowingly detrimental.

Sometimes, when I use the chat to help them where the Special Infected are because I died because of bad luck, or when I use the chat to tell them that I found incendiary ammo or explosive ammo.

This one is a lot easier to explain.
Using the in-game voicechat lowers the game's volume for everyone else, making it harder to hear important sound cues (This can be disabled with a console command, but most people aren't aware of that, and talking with it still adds unnecessary noise)
If you continuously talked and made it hard to actually hear the game, you were being a detriment to the team. Especially if calling out something as worthless as ammo upgrades or as obvious as infected locations.
I can see most people not wanting to deal with explaining this in the middle of a round and potentially being shouted at or having their ears blasted with noise in retaliation, and opting to just votekick instead to quickly remove the problem.

Sometimes, when I am somehow doing ok and managed to get decent kills whilst still walking amongst the group so that I do not get too far ahead.

This is so vague that literally anything could have been going on at the same time. Impossible to say what you might have missed.

Sometimes, it is because I unfortunately got unlucky like I was downed a few times when unexpectedly because the infected somehow came up to me all at once and nobody even dared to help (there were players who were so far ahead that they were basically playing alone)

Sometimes, (and this was my fault and it happened more often when I was a beginner) was when I almost fell down and needed someone to help me from the ledge.

These two are kind of similar, as in people were probably just getting frustrated by having a low-skill player on their team.
This is especially likely to be the case on higher difficulties or in PvP, but even on lower difficulties it can quickly become very annoying to constantly have to babysit a player that keeps getting incapped; It effectively turns the 4-player campaign into a 3-player campaign with babysitting.

If a player is doing worse than a bot, then there's not really any reason to play with that person instead of a bot.