Many transparent plastic materials, rigid or not, turn opaque (white) when subjected to stress, deformations or when folded (if not rigid) . The effect seems to be irreversible.
What is the cause?
I would have guessed that some internal crystalline structure is broken, but plastic (like glasses) have no crystal structure, no?
The process of forming an opaque area when you apply stress to a polymer film is known as crazing.
Polymers are long chains and, when coated, often form tangled, amorphous, randomly aligned polymer regions. When stress is applied to the film, the stress can be relieved by the polymer chains aligning. In other words, energy introduced into the film by stressing it can be used to align some polymer chains.
Increasing the alignment in the stressed region means that the density of polymer in this region of the film will be different from the density in non-stressed regions. This difference in density will cause a difference in light reflection and scattering accounting for the different optical properties between these two regions.
Further, within the stressed area of the film, mechanical stresses are produced between areas that are more and areas that are less aligned. These stresses can be relieved through the formation of micro-cracks between the regions. These small cracks also result in different light reflecting and scattering properties of the film.
Finally, as the polymer chains align, they are more likely to crystallize. The formation of small crystallites within the stressed (aligned) region will also alter the light reflecting and scattering properties of the film. Often, upon heating or standing at room temperature (for a longer time) the optical changes may reverse as the thermal energy supplied will:
References: The following links provide more information about crazing and associated phenomena at different levels of complexity:
@Philipp already mentioned microscopic cracks. This is called crazing. When a plastic is stretched, the extra space is filled randomly/irregularly. The gap between two polymer chains will grow, but most chains won't break. Also, if the material is already crystallized locally, the crystals will pull apart.
As the overall structure is non-crystalline, the cracks will have different sizes, and in particular quite a few will have sizes smaller than a micrometer - optical wavelengths, in other words. And with the variety in sizes, you don't get oen dominant color but a mixture of all colors which is of course white.
The other reason for the optical change is crystals forming. Again, these small crystals have different sizes around the wavelengths of visible light, so there's no dominant color from these either.
