Why does sunlight cause plastics to become a faded blue color? Due to the UV absorption, I knew that the plastics can be easily affected, especially PP. But is it normal that the change is always into faded blue from white color? and Why?

Because I made an experiments, but the results made me surprised. One sample of opaque white PP material was put on sunlight, one sample of same material having same conditions was put in lightproof black color bags. Then measurements was conducted every hour till 8 hours by using spectrophotometry which works in CIE Lab color space.

According to the results, b-value of sample exposed to sunlight had changed into the minus value, describing blue color in L a b color space, while L- and a-values are relatively stable. The b-value of another sample in lightproof bags had changed into the plus value, describing the yellow color in L a b color space, while L- and a-values are again relatively stable. So we can see that the effect of sunlight is only on b- value, which describes the values of blueness and yellowness of the sample.

My question is that what is the phenomena behind this?

What do you mean by reflected and transmitted lights? Even looked with naked eyes, easily seen that it becomes slightly faded blue color. The object is opaque white color PP material.
I don't think its actually blue, I think the blue is a result of Rayleigh scattering.

Over time the dyes are destroyed by light removing any color. simultaneously the plastic is degraded and becomes rough on the surface and acquires an inhomogeneous refractive index within the structure due to the destruction of bonds and oxidation and the end of chains. These two structural changes cause scattering, and since blue is scattered more than the other colors due to the relation of scattering with wavelength to the fourth power, the material appears blue when looking at the reflected light.  If you look at transmitted light going through the plastic it should appear yellow.