1.5 y/o GSD has started to become very defensive and protective.
I've owned Blitz since he was 8 weeks old. I've done all of his training myself and intend to keep it that way. I've made extra sure to socialize him with both dogs and people. He has never shown any sign of aggression and we always joke that "oh yeah he's a big mean vicious dog" because of how docile and submissive he is.
Within the last few weeks, he's shown aggression towards certain people. Each time it seems like it's when someone is approaching me, but only certain people. I can't figure out a common trait. He will run at them, barking his head off and snarling, stops right at their feet, and scares the living daylights out of people. He hasn't bitten anyone, but I'm not convinced that he won't, with this new behavior he's showing. I always go hiking with him off leash, and it's never been a problem. Very certain people will incite this behavior, and this weekend it seemed to be more common as he barked like that at two separate hikers.
It's happened a total of three times and I mean to put a stop to it ASAP. Does anyone have any advice? The reason he's doing this or a method to make it stop? Thanks!