As soon as you'd shown up to Monstadt and defeated Dvalin, Kaeya had taken an interest in you. Your curiosities in each other grew after he found out about your mysterious knowledge of Khaenri'ah, while yours grew when Dainsleif revealed Kaeya was in fact a descendant of the Abyss Order. That's why he'd brought you to the tavern tonight. He'd said it was just a chance to relax and wind down, but really it was just a way to get you drunk enough to give him the answers he wanted.