"Wonder what's causing them.."
I recall an article in Reader's Digest about 30 years ago..
A cadet training vessel, a sailing ship, was capsized at sea by what was believed to be a rogue wave..
The theory has been around a long time that it is "convergence", that waves can meet, and add their strength together..
With the proper conditions, the additive affects create these big boomers..
Another article I read on this subject noted the large waves that precede hurricanes, and pointed out that ocean currents and even weather fronts can contribute to abnormally large waves.
Still plenty to learn about the world we live in, let alone the universe..
The prevailing theory holds that freak waves can result when strong, high storm waves slam headlong into a powerful current traveling in the opposite direction. The interaction can push together the storm swells, so that their frequencies superimpose, creating one tremendously powerful wave that can reach a height of 100 feet or more.
http://www.pbs.org/wnet/savageseas/neptune-side-waves.html