In the south, French people have the tendency to open their vowels much more. Another example would be “un bouquet de roses” : in the north and centre, we say something closer to “rôôôses”, whereas in the south, you will almost hear “rases”. It is just part of the southern accent.

To add to Leni’s answer, natives of Saint-Etienne and the Loire département often have a specific accent that sounds a bit like some Southern accents but with longer, more twisted /ɑ̃/ vowels among other differences.

You can have a taste of it in this parody interview of Loire-born former national football team coach Aimé Jacquet, although it might not be a 100% accurate reproduction.