I've often seen in animated cartoons like Doraemon or any other anime that an object in the background that is about to move or do some movement is highlighted explicitly. Which makes it easy to predict that something is about to happen to the highlighted object. What is the reason for this?
This is due to how hand-drawn animation used to be made. A static background was drawn only once, and the moving objects were drawn for each frame on a transparent cel. Then it was placed on the background, one frame was shot on film, then the cel was removed, and new one was placed on the same background, next frame was shot, and so on.
If an object wasn't moving, it was a part of the background. If it is to be animated, it is no longer part of the background and has to be now drawn on the cel to be moved in each frame. Drawings on the cels, as seen on the picture are outlined with ink and that is why they seem highlighted or standing out from the background.
Source: Weiner Elementary, consulted on 27 November 2019.