How many hours do you have on the bindings? If there was a material or manufacturing defect, failure would likely have occurred in the first hour or two of riding. The most common cause for fastener failure is lack of proper preload. When was the last time you checked the tightness of the kingpin? In most applications, bolts are intended to only have induced tensile stresses; this requires a stiff joint, hence, proper preload. When bolts become loose, bending and shear stresses, as well as tensile stresses, are be induced in the bolt, leading to premature failure. Preventative maintenance is critical, even on something as simple as a snowboard binding!
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MikeC - sadly you are not the first person to have this problem and likely not the last. :(