Heartbreaking really. I remember a rescue dog, young male who became unpredictably aggressive. I think his owners were also expecting and surrendered him with a very heavy heart. At first it was thought to be behavioural and fixable, lack of training with a teenager kind of thing. I remember taking him out on leash to toilet and for walks and he seemed ok with me - no issues at all. He seemed fear aggressive if cornered in like say his crate so we avoided triggering that. But one day I was worki