Long-haired guy coming on from the right seemed to be signaling for a glove-touch, which is where the two fighters sorta do a friendly fist-bump at the start of the fight. It's sort of a sign of respect and sportsmanship, but it isn't at all necessary.
Instead of following through with the touch, though, hair guy went for a sudden takedown maneuver, hoping to catch his more sportsmanlike opponent off-guard. He was rewarded for his treachery with a knee to the face.