I read that there was an alert from an air traffic controller who saw the plane veer off course in his screen. Others said that the normal route should have been over the Pyrhanees, not the Alps.
The route varies due to weather/traffic...this link has previous days routes plotted, here is March 18 & they went over central France that day: http://flightaware.com/live/flight/GWI9525/history/20150318/0835Z/LEBL/EDDL