Can panic attacks just happen? or is one usually under some type of stress?

Panic attack: They are usually a combination of genetic predisposition and environmental stressors. Stress increases the start or recurrence of any psychiatric illness. The amygdala thru its connections with the brain stem usually causes symptoms of panic attacks. You can do meditation, carry a brown paper bag, when you have a panic attack, breathe into the bag for a few seconds, rule out medical and psych caus.

Spontaneously? Yes: Indeed, these can occur without any immediate or even recent external situational provocation. However, people who have had either chronic stress or an acute traumatic stress might be even more at risk for the spontaneous type as their bodies develop the stress response as a sort of habit. Remember that it is not life threatening even if it feels that way and do seek psycho/psychiatric help. Best!