To me, the term broach is when a boat inadvertently turns broadside to the wind or waves and the result is sometimes a knockdown. The knockdown could be caused by either the wind or breaking waves (worse) but the broach is a function of loss of steering for a variety of reason.
Broaching and taking a knockdown with a kite up is quite common amongst racers but being knocked down while cruising is a whole different ball game. Unless the conditions are extreme it is probably a result of poor seamanship (too much sail, badly balanced helm, broken windvane/autopilot)
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217