The only way yellow could be issued if the ball is booted while being out of bound already. Once it's on the field, on what basis anyone will issue a yellow?
Yeah, your guess is as good as mine, you'd have to ask the refs that issued the cautions. I can speculate that they deemed it "unsporting behavior" because they believed it wasn't in the spirit of the game. Refs have wide latitude when it comes to invoking spirit of the game elements.