Cracks always happen when the metal is under cyclic stress, but in this case the metal was weakened and perhaps embrittled by the galvannic corrosion. So which was it, stress or corrosion? The point is that you just can't tolerate corrosion on your boat! These boats don't have a lot of safety margins engineered into them. If they did, they would be thicker, stronger, heavier and slower, and we know we don't want that! So you just have to stay on top of stuff like this.

The anode is a gret idea, BTW. I think zinc is the standard metal used for this purpose and you can buy zinc anodes at any marine store. One for the mast and one for each beam. Sounds like a plan, especially for these boats with painted masts and beams. Adds a few ounces weight at most.

Jimbo