AS far as I can gather, it's just a big concert, and about people celebrating their home area of L'Occitane: the flag is that of L'Occitane: as you'll have seen from the link I gave to Wikipedia, it covers a big swathe across the south of France, and you'll maybe have seen from some of the Youtube comments, they have their own special language or dialect in that region. I will ask my French friends more about it.
(Similar things happen in Scotland when there's a big Scottish band on: everyone starts waving the Scottish saltire! - and, of course, we have the referendum on Scottish independence coming up next year. And you'll get Cornish flags in Cornwall, Breton ones in Brittany, Basque ones in the Basque country, etc)