Why is de added to the question?

Ayomide, the verb form that translates as ‘come from’ is ‘venir de…’… 

This is an inverted question form, and the preposition ‘de’ is placed before the question word (and hence the verb itself). It is similar in structure to the formal sounding English sentence ‘From where do you come (or hail)?’

Je viens de + [ville] = I'm from + [city] in French

https://www.wordreference.com/fren/venir%20de

 https://www.wordreference.com/enfr/hail