Can someone explain what happened?

So I was in a ranked match and whilst in champion select, someone seemed to be listing off what we were picking/choosing as summoners and what not in chat. But when we looked at the teams, they weren't on our team and when we addressed the person, they spammed chat with ":x" so we couldn't communicate with one another. Can someone please tell me what happened?

[NOTE]: I know my build is bad and so was my play, just wanting some sort of answers for what happened.