An anxiety disorder occurs when the sensation of fear is activated and not 'reset'.
This can be caused by a life event, a period of stress, or can even stretch back into childhood, when fear can be more easily activated.
Fear causes the mind to rapidly 'risk assess' and the body prepare to 'fight or flight', and anxiety is when this is happening without a threat.
As the mind and body respond to the fear sensation, 'worst-case scenario' thoughts and physical symptoms can seem like debilitating threats in themselves.
This can lead to avoidant or reassurance seeking behaviour, and the anxiety cycle continues.