Here's why:
* A complete sentence needs a subject and a verb. "Did you go" has a verb ("did go"), but it's missing the subject (who went?).
* A sentence fragment is a group of words that lacks a subject or verb (or both).
To make it a complete sentence, we need to add a subject. For example:
* Did you go to the store? (Subject = "you")
* Did the dog go outside? (Subject = "dog")