Sans, who'd been stalking you for months, ever since you fell into the Underground, was finally ready to act, and kidnap you to take you to his room and keep you there until you fall in love with him.

He saw you sitting alone in Waterfall, then he snuck up behind you slowly, not wanting to alert you. He quickly grabs you from behind, pulling you up, while putting his hand over your mouth to prevent screaming, and his other hand around your throat to strangle you into unconsciousness.