Looking a little closer at the fractured ends it looks like both props may have been cracked for awhile. The light gray areas in the photo are where the blades were still connected and the darker areas are where the cracks were. The reason they are darker is because the cracks allowed oil to seep into them. I've seen this type of failure in metal quite a bit and it looks very similar to your props. I doubt that this is a manufacturing flaw. I think the blades were stressed somehow.

RG