The picture shows the bottom of the mainmast, where the aluminum has been bent when the mast moved out of the mast step.

The mast has three pairs of shrouds holding it in place against side-to-side motion, the lowers, the intermediates and the uppers. The lower shroud, made of wire, broke. The intermediate and upper shrouds, were made of a synthetic rope rigging material called Dynex Dux that I'd installed in Argentina and Uruguay. Almost immediately after the lower shroud broke (we only heard one bang), the mast bent under the pressure of the sails and the checkstay, which was made of another synthetic rope called Spectra, bent the bolt it attached to the mast with and broke.

The pressure of the sails also came on the intermediate and upper shrouds, however, they stretched, and did not break. The stretching of the intermediate and upper shrouds, meant that the lower shroud, which was wire, and, to a lesser extent, the Spectra checkstay, took more of the load of the sails and broke.

The mast is stepped on deck. I had not unstepped the masts before, so did not know that the mast step is only a 1cm high aluminum plate that fits the bottom of the mast tightly. That small a surface for the step works fine, as long as the mast is held tightly in place by the shrouds and stays. The shroud breaking and the other shrouds stretching allowed the mast to move more than the step could handle, so the mast moved out of the step and got bent while moving.