I did some web search and here is what I found: Sunlight damages hair just like it damages skin and the lighter the hair the easier it gets damaged.

This is a photochemical damage to the hair shaft that breaks down the sulfur bonds and the hair becomes fragile. The hair pigments that are vulnerable to chemical modification by this photo-bleaching process lose color.

Rearrangements in the pigments molecular structure changes the way light is absorbed and reflected.

A similar effect can be caused by chemical bleaching by the hair stylist or from chlorine in a swimming pool by a process called chemical bleaching.