Randomly barks at my husband and other men
I have a 14 month old GSD male. He has had basic and intermediate obedience training, basic protection training and we just began Schutzhund. In the last few weeks, he randomly barks and charges my husband, as well as my father in law and my uncle. It is totally unprovoked, we could be sitting and talking or the men could be walking around the house. He has done that only to men, not women. It has happened at our house and my in-laws house. My husband corrects him if he does it and he settles down, but I know it's unnerving to others. After all, a GSD barking and charging is pretty frightening. He is friendly and has been well socialized since he was a pup, and this behavior seems unlike him.
Could you tell me why he is doing that and how to curb this behavior? Has your dog ever done this? Thanks in advance!