No clear trigger. You wake up in this super alert and dreadful state. Like how in movies you see the protag wake up from a dream that causes them to bolt up right and usually scream?
That's actually a night terror is. But without the dream. The dream part makes them nightmares. Usually, most scream while waking up. I don't, but still get a variety of the symptoms. Such as waking up instantly and the feeling of intense dread and fear, accelerated heart rate, sweating, and all that jazz.
I tend to have night terrors first, then when going back to bed (If I can.) A nightmare right after.