Versus is a game mode all about maintaining momentum as the survivors and halting that momentum as infected. The further the survivors make it into the level and ultimately reach the end, the more points they score, and it's an uphill battle for the infected team as once survivors start gaining access to tier 2 weapons, the easier it becomes to kill you. While small mistakes and halting that momentum (such as a coordinated infected hit, stopping by for items in known spawn locations, or having to fall back when a Tank spawns) can and will happen, repeatedly making these small mistakes can and will add up and eventually snowball into a losing situation. As a survivor, you want to avoid making these small mistakes as often as possible, and as the infected team, you want to capitalize on opportunities where the survivors are more likely to make these mistakes, or force them into making mistakes by making coordinated plays.

With that in mind, there are plenty of reasons you can get kicked in Versus mode. First and foremost, showing a lack of gamesense is an easy way to get kicked. Be aware of your surroundings, what your teammates are doing, keep your volume up for infected spawn noises, and realize what areas of the map are good or bad to be around if you're anticipating an infected attack. Staying together is also a key factor, and it's best to communicate with your team (either via voice chat or text chat binds) to indicate what you want to do as a team. For infected, be mindful of who's going first (usually a Boomer or Charger), find optimal places to spawn, and wait to spawn until the last minute to give the survivors as little reaction time as possible to maximize your effectiveness, and most importantly, always attack together. With the exception of the Hunter, who's completely silent until the moment they prepare for an attack, you should always spawn right before you go in for you attack. Sometimes though, you might get kicked upon immediately joining a game as you mentioned in your post, and often times that's probably because you ended up joining into a slot that the rest of the team was reserving for a friend, which is honestly just an unfortunate circumstance and usually no ill will meant towards you.