They can happen if the nose gets hit, or as in the case of my son, he had a vein that was very close to the surface deep inside his nose and it kept breaking and bleeding. He went to an ENT (ears, nose, and throat doctor) and the doctor cauterized (burned ithe vein shut with a special tool ) and the vein never bled again.
Agree with Sincerity..I would only add.. Dry winter conditions can contribute to sensitive inner lining of the nose to become dehydrated. Thus bleeding can occur. I use saline solution to flush and moisturize during dry conditions to prevent this from happening