Could he be picking up on some type of anxiety from you all (ie: worrying about how he'll react at the fence) and thinking there must be a threat?

Many GSDs are very empathic and will pick up on their people's emotions. Sebastian isn't so much but Guinan, Mozart, and Ozzie were.

Second thought is a stretch but... have you changed his food recently? If not, could you try him on a lower protein kibble of some sort? I've only ever seen it with Chopin but higher levels of protein in his food made him extremely aggressive - to the point where he even tried to attack me! We were even talking about having to have him put down when I read something about protein & aggression and we switched him to a very low protein kibble and he went back to his original cuddly self...

Thirdly, with the zapper. Are you issuing a verbal command before pressing the button? Always start verbal & then if he ignores you, give him a zap. If he stops & then reattacks, command again and then zap him harder/longer. Then again if needed. (If he hasn't gotten the point by a 3rd long hard zap, he's not going to so go on & catch him and drag him in.)

If he stops and comes to you, then offer praise/treat immediately. If you have to grab his leash & drag him to the porch, do NOT reward him at all. Only reward if he does what you tell him to do.

Fourth thought which would work in summer but not so much in winter. Instead of e collar, water hose. Yeah, that's not gonna work in the winter but if the problem persists into spring, it might be an idea.

This won't be easy to correct because a) GSDs are generally very territorial. "That's MY yard - stay away from it!" and b) He probably thinks he's protecting you from the other dogs. And I still haven't come up with a way to convince Freddie that he doesn't have to protect me from Sebastian... I have to be very cautious to not yell at or punish Sebastian in front of him.

Good luck.