Here's why:
* Adjectives describe nouns. In this sentence, "hurriedly" describes the manner in which she sat down.
* Adverbs modify verbs, adjectives, or other adverbs. While "hurriedly" modifies the verb "sat", it's not a typical adverb because it describes a quality of the action, rather than simply giving more information about how, when, or where the action happened.
Therefore, "hurriedly" functions more like an adjective in this context.