X-rays cause radiation damage primarily by their property of:

Biologic effects of ionizing radiation occur through direct and indirect actions. In direct actions, the photon directly interacts with and ionizes a biologic macromolecule. The free electrons produced by the ionization interaction (secondary electrons) may also interact directly with biologic macromolecules. In indirect actions, photons and secondary electrons interact with water and the products of water ionizations cause biologic damage. Direct and indirect actions both yield unstable free radicals—atoms or molecules with an unpaired electron in the valence orbital. Free radicals are extremely reactive and have very short lives. Free radicals play a dominant role in producing molecular changes in biologic molecules. Key Concept X-rays are ionizing radiations and ionisation is responsible for the biological effects or radiation damage. White and Pharoah's Oral Radiology Principles and Interpretation 8th edition