First night out, you haven't adapted to a a new sleep pattern yet. When you were off watch earlier, you weren't able to get any sleep because you weren't accustomed to the motion of the boat and you weren't all that tired and sleepy yet anyway. Now, you're alone on watch, it's dark and cold but you are bundled up so you can stay warm. You still aren't used to the boat's motion and it's a little difficult to move around so you pretty much plant yourself in a leeward corner of the cockpit. The excitement has worn off, you're bored and by now, getting pretty tired but you still make your horizon checks every 20 minutes and in between, you curl up in your corner with something to read.

Then, you nod off.

That's all it takes.

Like most disasters, there were at least two mistakes made. The first was setting the waypoint such that the boat was pointing at land rather than offsetting it enough so that the boat was pointing at open water and the 2nd was someone falling asleep while on watch.