There are several thing that cause to windscreen or canopy... There is crazing:When crazing occurs, each speck of damage to the exterior surface scatters the light waves in all directions, making it increasingly difficult to see through the windscreen.
Scratches, pits, chips and crazing create stress risers in the surface of the windshield, leading to the growth and propagation of the damage through the window material. The damage can reach a point at which it exceeds the aircraft manufacturer’s limit and the window will need to be replaced.
Certain chemical damage and of course lighting and bird strikes...