When you back out you are obligated to yield to oncoming vehicles.
When two vehicles back out they are both obligated to yield to oncoming vehicles so it becomes rather difficult to apportion fault.
However, this is how I view it (and I could be wrong).
A vehicle backing out into its own lane has only it's own lane to watch for.
A vehicle backing out across two lanes has both lanes to watch out for in both directions and certainly cannot look at both directions at the same time so when he's looking one way he's not looking the other way. And when he hits somebody because at that moment he was looking some place else, that's negligence.
Whether it's 100% negligence is another question.
Because when you back out into your lane you are looking to the right for oncoming vehicles and not looking across the street. If you pulled out nose first you would have been able to scan in all directions and avoid the car pulling out across the street.
I think there is a bit of negligence applied to anybody who backs out and hits or gets hit, simply because it's impossible to look in all the necessary directions. And when you hit something where you weren't looking, it's negligence.
Don't be surprised if there is a percentage of fault applied to each of you.