The development of panic disorder can be explained by classical conditioning. Classical conditioning involves pairing a neutral stimulus with a stimulus that produces emotional or physical responses. In the case of panic disorder, it can develop by pairing a neutral stimulus (heart palpitations) with an event that produces anxiety and panic attacks (being in places where help may not be available). After enough pairings, the neutral stimulus becomes associated with panic attacks, to the point where when you are presented with the neutral stimulus alone, it can lead to a panic attack. So panic disorder can develop from conditioned fear, where a neutral physical sensation is paired with a panic attack-inducing event, so that with enough repeated pairings, the neutral stimulus can then trigger a panic attack.
For more information on how panic disorder develops, please watch the following video: