What causes random nose bleeds?

The most common cause of anterior nosebleeds is dry air. A dry climate or heated indoor air irritates and dries out nasal membranes, causing crusts that may itch and then bleed when scratched or picked. Colds may also irritate the lining of the nose. Bleeding may occur after repeated blowing. When you combine a cold with dry winter air, you have the perfect formula for nosebleeds.