Epistaxis is the medical term for nosebleed. The nose is a part of the body that is very rich in blood vessels (vascular) and is situated in a vulnerable position on the face. As a result, any trauma to the face can cause bleeding. The most common way to cause the bleeding would be nasal membranes dry out, crust, and crack, as is common in dry climates, or during the winter months when the air is dry and warm from household heaters.
(hope this helps)