You don't really think of badminton as an overly aggressive sport.  But at a tournament in Vancouver on Sunday, things got heated . . . for reasons we don't know . . . and a player ATTACKED one of his opponents.

You can check out the video online.  What we DO know is that the two players have a history.  They're both from Thailand, and they were TEAMMATES at the 2012 Summer Olympics.

Then in January, one of them announced his retirement . . . which apparently didn't stick.