Large format Alum. they tend to break where the edge of the dome meets the compliance.That is were the Diaghrams are weakest or stressed the most. IMHO
The one time i've seen a broken large format phragm, this is what had happened to it. But, the dome had very evident marks where it had been in contact with the phase plug. My thought is that someone had driven it without any hi pass filter, or it had been rode really hard and left for dead.