One theory as to why regularly practiced relaxation techniques work is that our adrenaline level stays high so any small thing (and remember 'good' things as well as 'bad' things cause anxiety) can make the adrenaline level jump into panic attack range.

Regular breathing exercises (and hypnosis and guided imagery) helps keep the adrenaline level low or normal so that when something sneaks up that we might not even recognize as something that can cause anxiety, it won't send the adrenaline to such a high level that it causes a panic attack.

I do find that when I am able to practice regularly it's helpful but not sure if it is because of the above theory or some other explanation.

Just my thoughts.