Too much wave. Not enough boat. - It looks like a 22-26 footer, in 10+ foot swells under the Golden Gate Bridge.
I beleive that the wind was blowing into SF bay, and the tide was going out. This combination can create some nasty swells.
He *should* have had the companionway hatch closed if there was any chance of getting pooped. He may have stood a better chance if he were motoring, or at least had reduced his sails, (you can see that he actually gybes inbetween pictures 3 & 4) and if the weight were more aft. You can see the bow go under in picture 5. Also note that the transom has a cutout for motor, but there is no motor present in pic 5, or 26.