There are literally hundreds of causes of anxiety attacks. At times it can be really difficult to discover the actual cause of the anxiety attack, as there are often side effects and other symptoms which can mask the real cause.
In my case I suffered anxiety attacks because I was paranoid about not being able to get to sleep - due to work pressures. Initially it was really hard to tell whether I was suffering from insomnia or something else. It took few counselling sessions to uncover my actual cause.
Anyhow I wrote an article on understanding the causes of anxiety attacks.
Check it out here:
http://www.anxietyattackcures.com/Causes_Of_Anxiety_Attacks.html

