I think a lot of animals assess unfamiliar animals aspredators/non-predatorsby the look/build of them. For instance, most animals with eyes set in the front of the skull are predators. It helps with pursuing prey. Most prey animals have eyes set in the side softhe head, which helps them keep an eye out for predators by giving them almost 360 degree views. Leopards probably have a similar enough build to mountain lions, which deer instinctively fear, that they are cautious of leopards as well.

Just my theory.