What is happening?

The lens is contained in a sealed bag, called a capsule. As cells keep growing, they become trapped inside the capsule. Over time, the cells accumulate, causing the lens to cloud. As the cells increase in number, the lens becomes hard. The cloudy, hard lens blocks light and scatters what light does come in.

Why?

Diseases
Genetic factors
Smoking
The aging process
A previous injury to the eye
Prolonged exposure to sunlight