Here's why:
* Missing Subject: There are multiple places where the subject of the sentence is missing. For example, "you were sitting in a taxi wondering if you had overdressed..." and "...when you looked out the window..."
* Missing Verb: The phrase "when looked out window" needs a verb. It should be "when you looked out the window."
* Missing Articles: The sentence needs articles ("a" or "the") before "window" and "Dumpster."
Here's a corrected version of the sentence:
"You were sitting in a taxi wondering if you had overdressed for the evening when you looked out the window and saw Mom rooting through a Dumpster."
This corrected version demonstrates proper syntax, with clear subjects, verbs, and correct articles.