"You were shot, and the Bloodsinger inside of you didn't want you to die. Not dying, I believe, entailed waking up, sealing the fact that the two of you are joined in a pact of blood, and using your own inherent desire to stay alive to eliminate all of the threats in the area."

Hikaru deliberately holds off on a lot of details because he's certain that they could get to that later. They had all night; he could make time for it.