Because Okumura was told by his grandpa that if they destroy "the other world", he and his brother can turn back to human. And to open the gate, they need a lot of blood of the devil, so he sacrifice Rin. This is revealed in episode 22.
Yukio didn't know that Rin was going to be sacrificed. As you can see in the second last episode of Season 1, Yukio was surprised that Rin's blood was going and he had said yes because he wanted to protect Rin by turning him back into a human, but when he saw that Rin was going to die, he stepped in.