I think for a broach to occur the rudder has to stall if not the boat will simply round up. Burying the rail is not a broach. I have raced on boats where burying the rail does happen quite often. On these boats the stanchions and lifelines are removed, mostly for sheeting but a tertiary benefit is there is less drag when the rail is buried.
Agree, broach is caused by loss of rudder control, usually by stalling (equivalent of airplane wing stall) the rudder. I think this can also happen on some boats when they are on the ear so far that there isn't much rudder left in the water.
So your carrying your spinnaker in too much wind, you head up a hair, the boat leans over a bit, you try to correct back down wind but the rudder is just dragging thru the water, it doesn't work, so the boat keeps heading up until the rail is in the water or worse, and you cannot fix it with the rudder. You gotta get rid of some sail or it just sits there on it's side...rudder is stalled, ineffective.
Or, your surfing down a wave, you get going real fast, you head up a little, the rudder stalls, and you lay over on your side or worse roll the boat over...like surfing in over a bar. Again a rudder stall, the rudder becomes ineffective.
At least that's what I thought.