This is because they want to give distinction to the move that Pokemon used. Following your example, Tackle is the move Butterfree used, and to make it immediately visible to the player, it's placed on the next line. It stands out and is emphasized over the other words.
Other lines that do not contain any such feature they want to emphasize, such as "It does not affect Butterfree..." do not need a line break.
Hope I helped!