How can water pressure increase the factor of safety in Unwedge?

If the water pressure increases the factor of safety, then it is most likely associated with a change in the mode of failure.  

For example, if the wedge is sliding on one plane, then water pressure is added in such a way as to cause the wedge to slide on two planes, then the shear strength on the second plane can be enough to increase the factor of safety.  This is particularly the case if there is cohesion on the second plane.