Players who talk trash when winning, then lose.

I just played a game yesterday before work and during the game, this person would constantly put "?" In all chat after getting a kill or an ace. This person even went to as far as to say, "You know you can FF now, no shame in surrendering." In games I look at this and just sigh, I don't stoke the fire, or feed their ego, I focus on my game and my game alone. Here's where it gets weird though and I've come to notice this... When my team won a team fight, took baron, then aced them again which won us the game. Normally I don't like rubbing in a comeback because I won't lie, I was pretty garbo that game but... I said GG then. "Hey -Insert Champion name here- ?" To that, the person said, "You must be mad." Skip to the lobby and he goes on a tyrant of how I'm mad and that he is fine with the loss. What is the mentality of talking trash all game, then to lose and act childish about it? Stating before, I don't condone trash talking in retaliation to a comeback but I've seen this as a trend through all games, not only league.