Family Dogs Fighting Seemingly At Random?? [HELP]

Hey all,

I came here to get some opinions about our current dog situation in my house. We've gotten some advice from neighbors and dog trainers we know, but I'd like to see if anyone else has any ideas.

So our house consists of 3 dogs: a 12ish year old rat terrier, a 7ish year old chihuahua mix, and an almost one year old German Shepard. The problem we've been having is that the German Shepard has bitten the rat terrier (quite badly-requiring stitches) and has attempted to attack him once since then. The first time, it was obvious the terrier had growled at her (very lightly, just showing annoyance I think). The most recent time, he was doing nothing to provoke her. Before this incident (2 weeks ago,) they slept together, went on walks together, ect, with no problem. So for now, we're keeping them mostly separate and looking after both of them really closely.

Some background: the German Shepard has never hurt anyone else. She's an incredibly sweet dog who has attended dog training classes for quite a while and is fairly obedient and well trained. She's great with other dogs and has no problem with the chihuahua mix. When other dogs at the dog park growl at her, she backs away.

In contrast, the rat terrier has a history with growling at/attempting to fight other dogs (bigger dogs in particular). He was a rescue dog that we adopted when he was 3 or so and has always had issues and insecurity.

I understand the shepard fighting back when being growled at, although I wish that wasn't her first response. But the most recent time, he wasn't doing anything, and the attack just seemed completely random. It seems like a switch has flipped and all of a sudden, she just can't stand him.

When she was a puppy and the terrier would annoy her/steal bones from her, she would bark at him nonstop, and so we trained her not to bark at him. A friend suggested this caused her first reaction to switch from barking at him, to biting him.

Does anyone else have any insight about this situation, or any ideas for how to address it? Thank you so much!