Density of defect states are increasing it simply means that bonds are breaking and setting in irregular manner due to light exposure. Now your question should be why bonds are breaking and of course it will depend on many factors related to used material and u need deep study for specific material property.
On prolonged exposure of light, valence alternation pairs are generated and due to this creation of light induced metastable defects takes place. thus it is clear that on prolonged exposure of light DOS increases.