A few things. Most nosebleeds do not have an easily identifiable cause. Trauma to inside of the nose from nose picking very common. Other causes are exposure to warm, dry air for prolonged periods of time, nasal/sinus infections, allergic rhinitis, nasal foreign body (object stuck in nose), vigorous nose blowing, nasal surgery, deviated/perforated nasal septum, and cocaine use. See ENT doc for exam/treatment.