Why is buck shocked when fancois fastens straps and buckles on him

Buck is shocked because in his memory buckles (harnesses) were reserved for horses, working horses. 

François fastened upon him an arrangement of straps and buckles. It was a harness, such as he had seen the grooms put on the horses at home. And as he had seen horses work, so he was set to work, hauling François on a sled to the forest that fringed the valley, and returning with a load of firewood. Though his dignity was sorely hurt by thus being made a draught animal, he was too wise to rebel.