It's known that ionizing radiation such as X-ray and $\gamma$-ray, cause DNA damage, specifically double strand break. Why is it so? I mean, why not single strand break, why not pyrimidine dimer?
Ionizing radiation does cause single-strand breaks and other kinds of DNA lesions that are not double-strand breaks. However, double-strand breaks are the most difficult to repair and are thus the most likely DNA damage type to result in mutation and / or cell death. See Fig 1. from DNA damage response signaling pathways and targets for radiotherapy sensitization in cancer: