i confess i wouldn't have got out of a car on a motorway, what was she thinking. sorry that she has lost her life, but does seem rather strange. as you say we don't know the full details.
I think if you owned a pet that got into that situation you might very well do the same without thinking, same as those who jump in the sea and lakes for their pets. I think I would. Condolences to her family.
I think that people act instinctively in situations like this.
When dogs fall into water they have a good chance of survival due to their higher body temperature - but at the time you just don't think.
My rescue dog chased/annoyed a swan one day and as the swan came swooping towards him I got between them. Robbie was very brave, standing behind me barking his head off and someone shooed the swan away. But the bird was huge and would have injured me (not the quivering hound) with one punch (?) from a wing (or whatever they use)
I expected the swan might do exactly what it did do. It had been a bit off shore and Robbie had been running along the edge barking.
I should have grabbed him and dragged him off, or you would rather that I allow the swan to attack him and one of both getting injured.
My post was about my spur of the moment reaction - I called him back, he hid behind me and I attempted to protect him. It was a stupid thing to do - but nobody got hurt.
I got the impression from your post that you were more annoyed with the swan than your dog, who started it !! Of course I wouldn't have rather the swan attacked, what a silly thing to say ...