My cocker spaniel hates older men that do not have a dog with them and barks at them why?

my dog is very loving to children but when she sees an older man at the lake or park without a dog with them she starts running towards them barking,she never would bite them but still this is a nuisance. S he never expeirienced anything bad from an older man as iv had her as a very young pup and bought her off my aunty and uncle who love animals. is she protecting me?