No simple answer. The root of anxiety disorders is thought to be related to the "fight or flight" reaction gone awry. Anxiety itself is a normal emotion which can help us get through life-either by allowing us to act quickly in an emergency or in a mild form by keeping us alert. It is thought that this reaction can occur at inappropiate times leading to an anxiety disorder.
Many different ones. The roots of anxiety disorders are many and include exposure to trauma, a tendency to be anxious and shy, genetics and difficult upbringing even when no trauma occurred. There are occasions in which low blood sugar or some foods you eat can cause or worsen anxiety (like caffeinated drinks) but these are unusual.