Unless the conditions are extreme it is probably a result of poor seamanship (too much sail, badly balanced helm, broken windvane/autopilot)
And sometimes, no matter how good you are or how well trimmed the boat is, the waves get big enough with short enough wave periods that you actually fall off them! I was knocked down twice in one storm, The Perfect Storm (and not even near the bad parts), once by falling off the wave and the other time was by having the bow stuffed by a huge wave which resulted in a "broach" as you guys call it. Semantics aside when the mast goes in the water, for what ever reason, I call it a knock down.
In my experience you will:
1) Get hurt (it's hard not to when a 50 foot vessel is dropped like a toy boat) I've broken fingers been smashed in the head by a flying VHF and a wine glass and had my eye swollen shut from a stopper knot that was whipping around at light speed! If you want my advice wear a bike helmet & ski goggles when it gets that rough!
2) Not have time to be scared! Your too busy..
3) Will at times lose control of the vessel no matter how good you are. Even hove to you can take a knockdown. Displacement hulls are not designed to go 14 knots down the face of a wave and when they do they don't handle the way you might think.
4) The boat will right faster than you thought possible. Everything happens in slow motion except the boat coming back up and the water draining from the cockpit..
5) Things will be broken no matter how much thought went into being prepared.
6) You will most likely not be knocked down due to wind but rather short and steep waves (unless racing with a chute).
7) Fast building storms IMHO are the most dangerous because the wave periods are very short compared to slower building storms which tend to have bigger overall seas but are more spread out..
8) The forces on the rudder in this type of weather are astronomical! Even with wheel steering your entire upper body will be sore for weeks! This is NO place for wimpy rudder designs..
9) Lock the lazarettes!
10) You can make decent head way under bare poles but do give up some control!!! I find sail "slivers", such as two tiny feet of genny, work better than a storm sail in some conditions..
11) Lash the dinghy to the fore deck with ratchet straps and have good pad eyes for this purpose!!
12) Desitin! In this kind of weather the LAST thing you want is SWAMP ASS. Trust me you're already miserable enough! Gold Bond does not cut it with industrial grade, Ocean induced, salty swamp ass. An NO there is NO WAY to stay dry even with your gourmet brand Henri LLoyd or Musto gear...
13) Vaseline petroleum jelly. Put it on your face! Trust me on this one it feels like your being sand blasted and the Vaseline helps....
If I think of more I'll add it...
That being said I have not encountered real sever weather since the 90's!! Woo Hooo, cross my fingers, and knock on wood!!!!