I've had another close look at the photo. The left hand side of the wheel certainly failed in sheer as expected. Clearly, the subsequent collapse of the wheel cause the failure of the right hand side of the wheel, which appears on closer inspection to have a brittle bending failure. See how the rim appears to have opened up from the outer side, the fracture propogating back towards the nipple hole. On the other side, a crack has run along the rim terminating in a shear failure out to the edge, just as expected for a failure in bending.

I think there is some possibility that this rim was overly brittle. How much does the rider weigh and how many miles on the wheel?? I would get the wheel to the metallurgical department of the local Uni and have it examined.