One of those rollers roaring past the reef could have created a current that pulled the boat off course. Once broadside to the waves, it would have been difficult to straignten out before hitting the rocks.

Entering the Onset channel from the Hog Island channel at the SW end of the Cape Cod Canal is not just a simple turn. Once past the green and out of the canal current, there's a pull to port which can drag you into the rocks. I always prepare to turn to starboard, even though I'm really just heading straight to the next can. Not fun if you're not expecting it!