Intracranial hemorrhage
An epidural hematoma is a collection of blood between the dura mater and the skull.  ("Epidural" means "on top of the dura;" a "hematoma" is a collection of blood.)  Usually, injuries to the head cause epidural hematomas by tearing arteries in the dura.  Because the arteries in the dura are large, they carry a lot of blood.  If one of these arteries is injured, it can bleed very quickly, and an epidural hematoma can build up almost right away.