I've had it go both ways too. Sometimes trigger, sometimes not. There are times too, when a trigger is "invisible", but can be figured out in hindsight. Even with hindsight, there is one I am 100% positive had no trigger. It was long before dx. It slammed me down hard, fast and out of nowhere. I was having a really good time for -- not sure exactly, but easily months. It was the best time of my life! NOTHING bad happened. Then, BOOM! Game over. Total darkness. For months. At the time, I knew nothing of BP or triggers, but can assure you that I wracked my brain for the tiniest scrap to explain "why". Because, please oh please, there just HAD to be, right? But there wasn't, and it freaked me the hell out.

Some still come out of nowhere, but others do seem to have a correlation with times of major stress or change. (Magnitude of effect often underestimated (by me) until well underway... sigh.) Others can see it earlier, when I'm still saying, "Pfffft!"