Bouchon Bakery Rye Bread recipe

I just tried making the above bread. I followed recipe exactly using a Kitchenaid 600 mixer, KAF all purpose and pumpernickel flours with 65% hydration. The recipe calls for 20 minutes of kneading at # 2 speed. After about 7 minutes the kneading looked like it should be the end — with about 13 minutes left to go (!). I continued to knead at #2, and at 10 minutes the dough began to “unspool” off dough hook — was as if the kneading was in reverse. when I took dough from bowl it felt like a rock. I am still pretty  new at this (3 months) but neither I nor my wife (More experienced) has ever seen 20 minutes for kneading.  And the result was really strange. Any thoughts?