A Kill Chain or Killing Spree happens usually when a mass rush starts. By chance or skill, the player will kill another snake, resulting other snakes to lunge for it. More snakes run into the player producing more and more mass. This event can place the player high on the leaderboard.
To activate and survive a kill chain, you have to kill at least two large snakes in a row in the centre of the map and have very high movement accuracy even when being a large snake while moving fast.