I wish I knew, though if you've been tested, examined, and diagnosed as having a benign condition, I would not worry about
it. I've had great days, relatively free of ectopics, and been sitting, enjoying a movie on T.V. when wham, some bizarre cluster of ectopics or a short run happen out of the blue.
The have come and gone for me over the last 40 years, it's frustrating, and each time they flair again, I go back to the doctor's, just to be sure all is well.
Some are a response to caffeine or other stimulants, sometimes they happen more when we start to come down with a cold, some are from a lack of sleep. Adrenaline is good for kicking them off in a lot of people, so stress can be a cause.
A web page that's helped me a lot is by an Electrophysiologist, a heart rhythm specialist. www.drjohnm.org/2013/06/benign-pvcs-a-heart-rhythm-doctors-approach/
Among other things, he has this to say "If I had only one adjective for PVCs, I would use capricious. We can’t often find a reason why PVCs come, why they leave, or why they come back." Visit the page, he offers other suggestions for emotionally dealing with them.
I hope this helps to answer your question.