Killua was spying on Gon and Palm during their date. Why? Because he had to protect Gon. Did Gon know that Killua was spying on him? Obviously not. While Killua was busy spying Gon and palm in the aquarium, he accidentally bumped into you.

“Oh- sorry miss-.”

He froze, you looked beautiful, until he snapped out of it. He almost forgot about Gon and Palm, and how he was spying on them.