Panic attacks that are unexpected and come out of nowhere are the consequence of anxiety circuits that malfunction intermittently and catastrophically. The panic attack symptoms come from the fear circuits centered on the amygdala, which is the emotional part of the brain. When you have panic attacks, this amygdala-based circuit becomes hyperactive intermittently. So this amygdala-based circuit has bursts of hyperactivity, leading to unexpected panic attacks.
And the panic attacks are maintained and worsened when you start to anticipate when the next panic attack will occur. In other words, in between panic attacks, you have extreme worry about when the next panic attack will occur. This is called anticipatory anxiety, which comes from the prefrontal cortex, the thinking part of the brain. So this anticipatory worry may indeed 'prime' you to have another panic attack.
Of course, there still may be triggers for your panic attacks, and these triggers should be identified whenever possible, as the panic attacks can be dealt with more effectively if the triggers are known.
