A head injury is the most common cause of bleeding within the skull. A head injury may result from motor vehicle or bicycle accidents, falls, assaults, and sports injuries. If you're an older adult, even mild head trauma can cause a hematoma. This is especially true if you're taking a blood-thinning medication or an anti-platelet drug, such as aspirin. You can have a serious injury even if there's no open wound, bruise or other obvious damage.