why is anybody surprised?
You're combining a vicious oxidizer with a material that oxidizes easily. The fact that a physical fault probably *started* the whole mess is more important than a buttload of oxidizer reacting with a highly reactive metal that is freshly shattered and therefore not protected by an oxidized surface layer.
I'm guessing (SWAG) that the oxidizer hit the valve much like water hammer and simply shattered it from physical violence akin to what we imagine doing to the screaming child on an airplane. The oxidizer does its job, and promptly (in an extreme exothermic fashion) oxidizes anything it touches.
FFS, they're using a hypergolic fuel. Some mechanical part suffered a RUD and spewed shrapnel. The end result is far, far, FAR less important than the cause. Knowing that a match will burn is less important than knowing what conditions could cause it to. Especially when human lives hinge on whether or not that match burns.
As far as burst disks, they are a solidly reliable technology. Installed in a manufacturer-approved fashion, in a manufacturer-approved fitting, they will burst at the manufacturer specified pressure (within reason) every time. If your system is so touchy that the safety margin is less than the rated margin of the safety device protecting it.... you are an idiot and should let someone smarter design it.
At the very least, we can be thankful they weren't using a florine based oxidizer. Florine fires are really something to see. From a safe distance. In proper safety gear. Only once in your life. Because watching wood, steel, sand, concrete, and even a stream of halon gas burn really makes you re-evaluate the definition of "safe."