There's no one cause of a brain haemorrhage. Obviously, physical shock to the brain could be one cause (e.g. someone walloping you over the head with an iron bar could lead to bleeding in the brain). Far more likely, however, is a ruptured aneurism. An aneurism is a condition where part of a blood vessel swells up like a balloon. This is either due to a weakening in the walls of the blood vessel or due to an obstruction in the blood vessel itself. (The latter situation usually relates to the 'furring up' of an artery). Some aneurisms may just be related to chance flaws in the way the person's body is made. Other causes are high blood pressure or 'risk actvities' like smoking.
While a brain haemorrhage is not the same as a stroke (which always results in brain cells dying), the two are related. (15% of strokes are caused by a brain haemorrhage).
This BBC link will tell you more about aneurisms:
http://news.bbc.co.uk/1/hi/health/medical_notes/603437.s tm
As I've said, a brain haemorrhage is not always associated with a stroke but there can be a link between the two. This link, from the Stroke Association, explains how young people can be at risk of a stroke. (The link is relevant because many of the risk factors are related to the formation of aneurisms):
http://www.stroke.org.uk/information/younger_people_and_ stroke/index.html
Hoping this helps,
Chris