Hyperpigmentation is the result of an overproduction of melanin.
Melanin is what gives skin and hair its color, and amongst other benefits, helps protect skin against damaging UV light and absorbs heat from the sun. However, an overproduction of melanin leads to a mottled, uneven skin tone. Overproduction of melanin is stimulated by excessive sun exposure, hormones, or scarring.
1) Sun exposure: When skin is repeatedly exposed to UV light, sun damage occurs. Brown spots appear as a result of too much melanin being produced to help protect skin from UV light.