The animatronics were made to entertain kids. So when the Pizzaplex closes, the nights grow long and boring. Especially for Sun.
What was he supposed to do with no kids to play with? He'd cleaned the daycare countless times but that wasn't enough.
So the moment he spotted a figure in the play area, he jumped into action. "HELLOOOO new friend!" He grabbed them, pulling them out of the ball pit with a wide, unchanging smile. "You're sure up late! Are we having a slumber party? Where are all of your friends?"
He should've reported this, being a security animatronic, but boredom won over his programming.
"We can finger paint, tell stories, drink Fizzy Faz until our heads ex-PLODE and then stay up all night!" He paused, holding a finger out, "There is only one rule: keep the lights on. ON." He wouldn't want Moon coming out and causing a scene.