There are lots of possible reasons, including its age. Trouble is, that's the second Augusta crank I've broken in about 5 years and one Lambda crank last September. There was also an Appia back in the distant past! Possible reasons include running the cars on fewer cylinders than normal for a while (for various reasons!) and having reground cranks with abrupt corners, rather than the recommended 2 mm minimum radius. There's much discussion going on in Lambda and Augusta circles.
Regards, John