My dog (1y) doesn’t like old men???

Hi, my dog is a little over a year old and we have had her since about a month and a half old (foster fail). Since about 6 months old she has shown that she doesn’t like old men - we don’t come across it much maybe like two to three times an until this morning. But whenever she sees an old man (specifically with white hair) she barks like crazy and does not let them pet her. But she lets anyone else pet her. Anyone have any idea why that might be?

She is mostly Great Pyrenees 60%, German 20ish% Shepard and border collie 20ish% mix.