Here's why:
* Proper nouns are names of specific people, places, or things. They are always capitalized. Examples include: John, London, Earth, Amazon River.
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are at the beginning of a sentence. Examples include: boy, city, planet, river.
"Down" is a common noun that refers to a general direction or a state of being lower.