Zip,
Not sure exactly what the root cause is yet, & I'm waiting 'til I'm back home to really dig into it, but basically the nut on the eccentric adjuster wasn't clamping down. I had torqued it to spec after adjusting the chain the night before, but I was tightening against corrosion or cross-threading or something & it was actually still loose. Suddenly having your chain gain 2" of slack at 80MPH is no fun!
Cheers,
-Kit