The cause is not completely understood. Snow nose is more common in light and white-coated breeds. This condition by itself isn't a health problem, but caution is advised in sunny weather, as the depigmented area is at increased risk for sunburn. Some vets advise testing the thyroid levels to make sure that there isn't an underlying thyroid problem.
Other things to consider
If your dog uses plastic food and water bowls, a change in nose color may be a reaction to the plastic. Try switching to glass, ceramic or stainless steel bowls.
Several conditions will cause a loss of pigmentation on a dog's nose. Thankfully, snow nose is not a health concern, but any changes in your dog's nose, including crusts, sores, discharge, sneezing or wheezing should be checked as soon as possible by your veterinarian, as they can indicate serious disease.
Note: I used a photo of my dog's nose to illustrate the typical location for snow nose depigmentation. Sophie is ten years old now, and this is a permanent coloration for her.