I went out to the garage after work last night and looked at things a little more closely. After closer inspection, I found that the last 3-4 threads on the center stud (the only threads I could get the mounting nut to grab onto) are totally sheared off, so it was in fact an explosion that sent it flying off, not just the nut falling off and the cover plopping out of place. Also, it turns out that my rotor wasn't seated truly on the crank and had a
tiny 1/16" wobble to it, almost not even noticeable. After I removed it I found 2 problems: 1, the center hub was not completely flush against the rotor, but ever so slightly. And 2, the mounting stud that the rotor mounts to the crank is so weak that the threads on the crank end are ready to snap off of the stud. The stud was bending where ever I moved it to with little effort.
Oh the joy.
Scottie