Interesting!

I'd guess it's diterpenes in coffee (cafestol and/or kahweol) having an effect on insulin signaling within skeletal muscle cells. I ran across a study indicating that they'd found a diterpene derivative that enhanced insulin signaling in specific (hepg2) liver cells. Kind of a leap here, but it's suggestive; diterpenes clearly are bioactive, but there's not a lot of clarity around the cellular mechanisms involved - not that I'd recognize an organic reaction if it bit me :-)