Within time cliques form and some people may be coming to those events out of fear they'll be missing out instead of to have fun and some people may just not be coming and will be viewed by other team members as weirdos. From there it is natural to side with something you know and feel affection to and you may be unconsciously treating people who don't socialise with you worse than the others. Other things that may happen, often when also alcohol is involved, people may say things from their private lives that then other people may use against them or when you start talking about politics and it turns out some people have completely opposite views to yours. This is not helpful in any way.