Rob said what I was thinking. Something is wrong with the way the wheels meet the hubs. If the wheels are centered to the hubs and the wheel center holes are too large, the weight would have to be borne by the studs alone.
Tandem trailers put a lot of strain on wheel/hub/axle components anyhow. Just look at how the tires "scrub" during tight maneuvers. Surprised, though, that the failure happened on the passenger side. The driver sides of vehicles used to have left-handed thead on the wheel fasteners because the left side tends to loosen rihgt-handed thread. Most of the time, if there's going to be lug trouble on a trailer, it'll start out on the left side.