Maybe they lost their balance or were pushed by someone. It could be a funny accident or part of a crazy storyline.
Maybe he lost his balance or tripped over something.
Maybe he was running too fast and lost his balance.
Maybe he was being clumsy or not paying attention to where he was going.
Maybe it lost its balance or was distracted. That could cause it to look down and then tumble.
Maybe he slipped on something or lost his balance.
It could be that they heard something downstairs and are curious to investigate. Or perhaps it's part of a dare or challenge among them.
It could be that he's being chased and is trying to escape. Or perhaps he just felt like running down the stairs for fun.
Maybe they are up to some mischief or playing a prank.
Maybe the character is sad or disappointed about something.
A cartoon character might look down because they've made a mistake or are facing a difficult situation. Or it could just be a pose for dramatic effect to draw the viewer's attention.