For the non-english speakers, You in french can either be translated as "tu" or "vous", you usually use it to designate/adress a group of persons, but you can also use it to address a single person, depending on various factors including the numbers of people, social position, rank in the army/job, closeness, age, personnal respect, etiquette or just how polite you are regardless of the previous factors.
If you use the "Tu" on a regular basis with someone, but you switch to "vous", it usually means either the discussion goes to official/job mode, or you've done something bad and pissed him off and he/she distancing himself/herself from you.
Nowadays let's just say it's less important to misuse the "tu/vous" thing in society as even 40 years ago.
But here, in this context, big deal.