There is an article about it in French here.

Basically, it's a sulphorous smell. This is brought about by the nature of the ground under Paris (largely composed of chalk and marls in the parts of town the line goes through, and its position relative to the water table. Remember that a lot of Paris is built on dried marshland (hence 'le Marais'). The extent of this was underestimated at construction time, and works are ongoing to improve the water-tightness of the concrete used, particularly in the stations.

There is a long-standing problem with the smell in the Paris metro in general, thought to be in part due to a type of bacteria found in large quantities in the Paris soil. There have been several experiments with various forms of deodorisation in stations.