Nosebleed Causes. Most commonly, trauma to the nose triggers a nosebleed. Trauma to the outside of the nose, such as a blow to the face, or trauma inside the nose, such as nose picking or repeated irritation from a cold, can cause a nosebleed.Less commonly, an underlying disease process, such as an inability of the blood to clot, may contribute to the bleeding.osebleed Symptoms. Bleeding usually occurs from only one nostril. If the bleeding is heavy enough, the blood can fill up the nostril on the affected side and overflow within the nasopharynx (the area inside the nose where the two nostrils merge), spilling into the other nostril to cause bleeding from both sides.