I do have panic attacks that I don't always know the reason for. It usually goes back to PTSD if I can work my way back. However I used to have really bad panic attacks in my sleep with no known triggers; I'd be asleep and then I'd be awake and in a full-blown panic. My therapist had me try some cognitive stuff and those improved. The panic attacks without cause often come when I should be relaxing or am already relaxed and then boom! panic. They don't happen very often anymore after working on it in therapy but they are definitely no fun at all.