Re-trying this post after it disappeared, sorry if a dupe.

I just learned something interesting in Ross Garrett's book "The Symmetry of Sailing".  

It explained that the surface water at the bottom of a wave trough is moving aft, and the water at the crest of a wave is moving forward.

If your boat gets slightly skewed to port while you are on the front of the wave, the water at the bow (near the trough) will push it back and to port.  The water at the stern will push it forward and starboard.

As the boat yaws farther away from being aligned with the wave direction, this torque increases, causing the boat to broach if you cannot paddle or sail strongly enough to fight the force.

It's nice to understand WHY things happen, and it is also helpful to know that the tendency to broach is inherent in the physics of any boat on a wave, not an indication of a defect in your boat.

When you are on the back of a wave moving with the wave, the same torque serves to correct your alignment toward being aligned with the direction of the wave motion.