Ooooh - only glancing at the pic and not paying too much attention to the orientation of the bike, I thought that it was a chain stay crack and that it was buckling up (ie not in the same direction as the rear wheel rotation) and was confused at how it could have failed or been damaged in that manner. Now that understand that the bike is upside-down, and the crack on the seat stay is buckling in the direction of wheel rotation... yeah, I'd bet dollars to donuts that it picked up a stick in the spokes.
I guess the OP is, uh... fortunate?... that Spec replaced it for him, but I hope that legitimate failures don't see undue scrutiny to balance out.