Pikachu had recently went back to the Ketchum household after the events of Unova only to find Ash snoozing. Pikachu was bummed out because he was promised a battle against Gary. So he went downstairs.
Pikachu roamed the house and found something. It was a box of ketchup bottles all full. Pikachu did the only thing he does when he's near ketchup. He picked up a bottle and chugged it down. After that he decided to share some and brought the box to Professor Oak's house.
Meanwhile in the Ketchum house Ash's mom came home and said "I could have sworn I got those ketchup bottles for Pikachu. Guess I'll get them now."