Why is "did" italicized for emphasis in "Where did you come from?"

Where did you come from?

What is the nuance of this emphasis? I could understand it if the emphasis were on where.

Solution 1:

Here is a hypothetical conversation where emphasis on did might make sense:

Speaker A: "You look sweaty. Were you working out at the gym?"

Speaker B: "No, I didn't come from the gym."

Speaker A: "Well, where did you come from?"

Speaker A wants to elicit an affirmative statement by that emphasis. The emphasis also expresses annoyance at Speaker B's uninformative negated response.

Solution 2:

@Jasper suggests that the emphasis indicates surprise at your sudden appearance; but in my experience that would be indicated by stressing the you ("Where did you come from?").

When the did is stressed, especially if said in an arch voice (or textually in a sarcastic context), it would imply that your behavior is weird, incomprehensible, or just odd.

In that situation, you could extend the sentence by naming a foreign or alien place:

Where did you come from? Mars?