Why might water pressure cause a decrease in Factor of Safety as the slope angle decreases?

Users may see this type of result when doing analyses where the slope dip changes but all other parameters remain constant. 

As the slope angle decreases, the wedge weight decreases and you get sliver wedges along the surface of the slope.  Since the maximum water pressure is a function of the height of the wedge, it doesn't change much as the wedge weight decreases.  What happens is that the water force approaches the wedge weight and the effective normal force on the failure plane reduces, and the Factor of Safety reduces.

 A good article pertaining to this is “A Slope Stability Problem in Hong Kong”