Here's why:
* Adjectives describe nouns. In this sentence, "hurriedly" describes the manner in which the subject sat down.
* Adverbs modify verbs, adjectives, or other adverbs. While "hurriedly" modifies the verb "sat," it's acting as an adjective in this case because it describes the action of sitting down.
It's a bit tricky because "hurriedly" is usually an adverb, but in this sentence, it's functioning as an adjective.