I'm not 100% about canards but from what I know about wings.
Dihedral (upward angled) wings contribute to stability. Often found on passenger a/c.
Anhedral (downwards angled) wings have more maneuverability. Found in museums on the Harrier.
Maybe with the Rafale, they wanted the back end to be manoeuvrable and the nose to be stable?
The physics are explained here
http://en.wikipedia.org/wiki/Dihedral_(aircraft)