Yeah, basically, what TypelA said in their answer. 

But then there is also the issue of grammar, I think. In "crouching tiger hidden dragon", both "crouching" and "hidden" are adjectives; they are modifying/describing the beasts. "Hiding" wouldn't work here, because that would sound as if it's being used as a verb (i.e., someone is purposefully hiding a dragon instead of the dragon hiding itself).