Why is my puppy scared of other people?

Me and my Fiance adopted a 6 month old border collie mix from the Humane Society from here and he didn't have problem with me or her and was perfectly comfortable with me. Well sometime after we brought him home. He constantly growls and retreats from my fiancee's brother and other stranger's but totally fine with me, my fiancee and everyone else in her immediate family. At first I thought it was just men in general besides me but I noticed he does it to some women also. Why is this and why so sudden? His only 8 months old now and he was a stray at one point in time. He also never bites he just simply growls at people and runs to me and looks for protection.