I have ribs do this on me, both wrapped and unwrapped, although much more often unwrapped. It was not just the heat. The meat on the surface of some racks can toughen, like jerky. I experience this problem far more often at lower temperatures too. I find that ribs that are not well trimmed on top do this most often, especially if there is a thin layer of meat, over a thin layer of fat.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."