My guess is that backstay or backstays failed first.

If the stay system is intact, then the failure would have revealed a different "look" from the fractured portion of the mast. The initial pictures of Archangel on the "rocks" dont look like a classic compression fracture of carbon fiber -- they look like a bending failure.

A mast has to break somewhere if it's going to break. In this case its a keel stepped mast that has a certain amount of stiffness. The deck partners and the keel step would have provided additional "stiffness" above the deck. (About 1/3 of the distance from the deck to the mast step)... Of course that's just my guess.

The boat was under sail and the rig came down to the port side of the vessel (after, in my theory, the backstay system let go. As you notice, they have a split backstay system .

If the backstay is "loose", but the main is brimmed in and "tight", then you can get a magnified "snap" when the loads jump from running into the back of a wave. If you run into a ledge and "DEAD STOP", you can get much more, I would guess. Even so, I wouldn't think that the rig should come down -- but, that's just my view.