What I read was that the lip of a hub centric design is what supports the up and down motion of the wheel (well, the weight is rested on that lip) whereas the lugs are what provide the lateral motion support. What surprises me about your situation, is the lugs coming away from the hub, not the spacer. If the lugs between the spacer and the wheel separated, then I could understand since the wheel would have no lip to support the forces, but since the vehicle's hubs were 'hub centric', the spacer should have been fully supported by the hub's centric part, thus that shouldn't have broken away at that point.
I take back my "makes sense to me" comment...it does not make sense, if the failure was at the lugs between the vehicle hub and the spacer.