The boy might push the girl accidentally or it could be a part of a humorous scene. Without more details about the cartoon, it's hard to say for sure.
Well, it could be that they're having a friendly chat or maybe working on a school project together. There are many possibilities depending on the context of the cartoon.
It could be a funny or cute scene, maybe part of a comedic or romantic storyline.
It could be a way to show a character's personality or create a conflict. Maybe the boy has a reason for doing it that will be revealed later in the story. Or it might just be a random action for visual interest.
It could be a popular anime theme song. Maybe something from a romance or adventure anime.
Maybe she's trying to get fit or build strength.
Maybe it's a way to show his affection or it could be part of a story plot. Who knows?
Maybe he's not paying attention or doesn't know it's a pull door.
Perhaps it's to introduce a conflict or a lesson. Maybe the girl needed to face a challenge or overcome a fear related to water. The reason really depends on how the story unfolds in the cartoon.
She might use proper form, starting with hands on the ground and keeping her body straight as she lowers and raises.
Maybe he's having a bad mood or there's some misunderstanding between them.