While the former is the stem of a na-adjective, the latter is a verb.
e.g.
年末の街はにぎやかだ。
年末の街はにぎわう。
にぎやかな場所
にぎわう場所
Incidentally, the etymology of them are the same word にぎ, which is considered a mimetic word to express "rich". There are other words of the same etymology such like にぎにぎしい or にぎわしい.