Toxic on Instinct

So I was playing rank and our team snowballed super early. Each lane was winning, ganks were seen ahead of time by excellent ward placement, punished enemy's early mistake and took a dragon relatively early in the game. We won the game super fast, but all throughout the game our top laner (Malphite) keeps trashtalking us for things that made no sense at all. For example, when we took an early dragon, he wandered into the enemy jungle and was met by the whole enemy team and from then on, he just started finding a reason to be toxic like "omg my team so useless" etc.

Has this happened to anyone else before when you're winning so hard with great team cooperation and peace but there's THAT one guy spewing toxic nonsense? xD