Flingwing is right, I think. The arrangement reduces the "propellor moment" which is the resistance to feathering that the blades present to the pilot when he pushes the left pedal.
The forward sweep increases the stress on the blade roots, if the blades were lagged, ("pre-lag") this would reduce the stress, as it would align the blades with the line of force they experience. This leading sweep increases the bending at the blade root.