[willy]

Exactly what was said above JJ, plus I would add this, when you turn and run it is an instant trigger for the prey drive in dogs to run after and to take down what ever it is.
I have had to deal with this stuff my whole life, had to shoot one German Shepard that attacked me and took me to the ground. Had to hit another one with a outdoor chair while yelling NO at it which caused it to back off.
One of the Officers I worked with had gotten bit twice in a month and everyone called him Milk Bone for years.
Sh!t happens, but what these guys above had said is probably what is happening, along with the circumstances you put yourself in, knowingly or unknowingly.
My current job puts me in situations where I am often going to peoples homes to post tenancy hearing notices, often out in the country in more rural areas. I have learned to look for the tell tale signs like a dog bowl, chain on a tree etc. Also if I just sense something is not right before I step away from my car I will whistle a few times or maybe beep my horn a couple of short beeps and listen before stepping away. That has saved me a couple of times.
If I think there might still be a chance I will have a pepper spray in my hand as I walk up to the door. But with my age and general disposition I will not be a milk bone again and have the dog walk away, if I don't have my pepper spray I will introduce Mr. K9 to my personal friends Mr. Smith and Mr. Wesson so fast the Duke would smile