Why/how does exposure to noise cause cochlear hair-cell loss?
There are a number of pathophysiological mechanisms that are thought to underlie noise-induced hearing loss:


Mechanical damage to the delicate cells and supporting structures of the organ of Corti;
Reduced blood flow to the inner ear;
Intense metabolic activity, which increases mitochondrial free radical formation.


Reactive oxygen species (ROS) are highly reactive. They are essential for mitochondrial function to generate energy. However, too many of them damage cellular lipids, proteins, and DNA, and upregulate apoptotic pathways. The observed impaired blood flow to the cochlea can enhance the toxic effects of ROS. Mechanical damage to the delicate hairs and membranes of the hair cells reduces their ability to converge acoustical energy into potential differences.

References
- Le Prell et al., Hear Res (2007); 226(1-2): 22–43
- Kurabvi et al., Hear Res (2017); 349: 129-37