Here's why:
* Tied is the past participle of the verb "to tie."
* Down is an adverb that modifies the verb "tied."
* Together, they form a verb phrase that describes an action: to be restrained or restricted.
The phrase could also be considered a past participle adjective if used to describe something. For example: "The dog was tied down to the porch." Here, "tied down" acts as an adjective describing the dog's state.