Arms Race - Level Up when killing specific people

Arms Race in Counter-Strike Go - is a certain game type in which you kill players on the opposite team to cycle through your weapons to reach the golden knife and win the game.

Every 2 kills, cycles a weapon to the next.

Every Knife kill automatically cycles your weapon to the next.

But I've noticed that when you killing certain enemies it sometimes cycles your weapon automatically. How exactly does this happen? Is it a head shot or a specific weapon you use or what?

Thanks