Radiation caries occurs due to all of the following except:
Radiation Caries
Radiation caries is a rampant form of dental decay that may occur in individuals who receive a course of radiotherapy that includes exposure of the salivary glands. Patients receiving radiation therapy to oral structures have an increase in Streptococcus mutans, Lactobacillus, and Candida.
Caries results from changes in the salivary glands and saliva, including reduced flow, decreased pH, reduced buffering capacity, increased viscosity, and altered flora.  
The residual saliva in individuals with xerostomia also has a low concentration of Ca2+ ion; this results in greater solubility of tooth structure and reduced remineralization. Because of the reduced or absent cleansing action of normal saliva, debris accumulates quickly. 
There is also growing evidence that radiation has direct effects on teeth that make them more prone to breakdown with flaking of enamel, particularly in areas of occlusal loading or stress, such as at the incisal, cuspal, and cervical regions of teeth. The destruction is seen with doses greater than 30 Gy and is pronounced when the teeth receive more than 60 Gy.
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 22
Bone defects
Fibrosis of salivary glands
Decreased flow of saliva
Low pH of saliva