Rusty's history. From the point from where we adopted him, and onwards (although I would love/hate to know what happened to him before that).
When we met him at the RSPCA, he was very skittish and jumpy. There was no aggression in his history that we could see or were made aware of, and I believe I remember seeing something at that shelter saying that unfortunately dogs who have bitten are put to sleep. They said he did have a history of digging (no longer an issue, he basically just grew out of it), and jumping at fences (we've never actually seen evidence of this, even when he is tryign to get at something outside, there is no jumping at the fence). My understanding is that his last owner had surrended him, as he was too much for them to handle. I beleive it was an owner (no idea how many he has had) before this who was the abuser.
He was pretty good in the car on the way home, but very quite, where as now he loves going in the car, and gets very excited.
When we got home, still very timid, and preferred to spend his time hiding in the garden (we basically never went in there, staying more on the lawn coaxing him out, so he would consider that his safe space, and is still his favourite spot to be when I take him to mum & dads).
It's really hard to say when exactly his aggressiveness first showed up, particularly because, as first time dog-owners, we didn't know what signs we should be looking for. He did obediance class not long after we got him, and while he wasn't great with other dogs, he was much better then he is now. Now that your making me think about it, it is possible that he has been worse with dogs as a direct result of wearing the muzzle, possibly in reaction to knowing his main defence is no longer an option.
I do clearly remember the first time he bit someone though. He nipped a jogger going past while we were playing fetch off-leash. We didn't let him play off-leash from then on, with the possible exception of large areas with no one around, and we would go in the middle so we could see anyone approaching before they got too close. The second time, he bit me, I was dragging him away from another dog he was reacting too, and as I pushed him with my leg to move (I am now aware this is not ideal, but didn't have any better knowledge at the time, and, honestly, he was too strong for me to handle appropriately, so I just needed to have us both away from the situation), he turned around and bit my leg. I believe this is in reaction to not really knowing it was me in his rage-fit, and he let go pretty quickly. After that, he didn't leave the backyard without the muzzle.
As for when he started barking at things passing by, I am not sure about that one. He would bark at very specific things when he was at mum & dad's place, but never (as far as we know) to the extent he apparently is now.
His aggression towards the family has never really been an issue. He bit me once (as above), and nipped dad once (dad had started the lawnmower in the backyard, which he hates, however this time mum & I were also in the backyard, so I think he was protecting us). Other than that, he loves me and mum, double checks who dad is each time then loves him as well (dad actually spends more time with him then mum, but by virtue of being male, there is a moment of cautious while he remembers him, and it automatically downgrades him a place in "who he loves most"). With my brother, he pretty much follows in my footsteps there, he tolerates, but generally ignores, him.
Basically, the aggression increased the his confidence and comfort with us. It wouldn't surprise me if there is some protective to us element to his thought process, however I am sure self-preservation/perceived threat is the dominant issue.