For a lot of children anger is a sign of sadness or anxiety. Although they may not always be able to explain how they feel, it may be worth looking at the pages on anxiety and low mood as well. Anger and aggressive behaviour can be hard to cope with, both for the child themselves and for those around them. These resources give some advice.
Other sources of help can be found in the Parents Survival Guide
A discussion on The Parents Lounge (YoungMinds) on coping with angry behaviour.