No one really knows what causes them - plenty of theories but not facts - and that's why they are both exceptionally facinating as to how they form, and yet also utterly terrifying.
I may have experienced a very small one, 10-15 years ago - and I still have flashbacks.
Was a crew member in a sailing regatta and for one 5-10 minute period the waves were just coming from everywhere, breaking all over the place. It was exhilarating, like riding a roller coaster up and down! Then a (relatively) very big one, out of the blue. It seemed to go between us and the next boat, maybe 60 feet away. From the deck, we couldn't even see the top of the mast of the other boat.
And then we fell out of the water.
Our boat, all 30,000 pounds of it, dropped straight down and we crashed, hard, into the water below. The mast bent so much that the cable stays were loose. The impact threw everyone down. The sound: it sounded like the fiberglass crunched and broke. For a moment we all thought we were dead. But we weren't. The mast snapped back so hard the cable stays made a twanging sound. Sails limp - no wind at the bottom of the hole. Then the water all around us swallowed us up; our buoyancy sent us shooting straight up with a lot of force. The wind hit us hard; no time to reflect; have to deal with that now.
All those years of racing, including being in multiple collisions, taught me that it is surprisingly difficult to sink a boat. As long as you can maintain hull and deck integrity, you need a LOT of water inflow in a very short amount of time to eliminate buoyancy.
We were never a good team, but I came away from it with far more confidence being on the water than I started :)
This is advice, hard won, by mariners who abandoned their ship, were lost at sea in their lifeboat, and left behind them a ghost boat that eventually makes its way to shore.
”[T]hree years past, there happened to me an event such as never happened to mortal man -- or at least such as no man ever survived to tell of -- and the six hours of deadly terror which I then endured have broken me up body and soul.”
A Descent Into The Maelstrom, Edgar Allan Poe, 1845
I have a theory that there is something down there, deep in the uncharted depths of the oceans lurking and causing rogue waves, but its just a theory one I dearly hope is wrong...
Even worse, it's standing in our night sky, in periodic plain sight, taunting us with it's waxing and waning, knowing there's nothing we can do to stop it.