That's a very good question! Is open an adverb or a verb? (Psst. It's definitely not an adjective in I pushed open the door.)
Note that, push open, meaning to open something by pushing, is a verb-verb compound semantically, with open as the head of the phrase and pushed as its modifier. However, pushed carries tense, which makes it the main verb and the head of the phrase syntactically. Thus: if pushed open is interpreted as a phrasl verb, open becomes a modifier. Why?
 I pushed open the door.
 I pushed the door open.
All the best.