Sometimes they just do. It's due to a loss of pigmentation. We have two dogs, who are closely related - one has a really black nose and the other, who is slightly younger has a pinky/black nose which is losing more pigmentation as he gets older.
If a show dog starts losing nose pigmentation, it's not unusual for the owner to apply iodine to the nose to darken the the colour. Apparently this causes some discomfort to the dog though.
Apparently there's a virus that dogs can get that makes them lose pigment... this happened to our dog, quite suddenly. One day her gums and lips were black, the next day they were pink. No adverse affects or anything - just interesting.