Realistically, no one would shoot arrows at you from behind, as that would mean your own people would be firing at you, and, no one on either side would fire arrows once armies are engaged. It'd also make it very difficult to fight with a heavy 2-handed sword with a shield attached to your back, as it'd hinder your swing. And realistically, it'd take you some time to get your shield off your back and square with it, so you really can "raise your shield fast" if it is on your back.
The shield would be on your back to be able to get it off for blocking purposes.
There are arrows fired even when two armies are engaged
A shield is probably not going to hinder your swing if it is on your back. Think about it. It is the practical way to carry a shield unless you have it on your arm for some reason.
A person who knows he is going to be in battles can have his shield ready to go from his back fast, it certainly does not have to be a slow process getting your shield off your back to block with it. Plus the raise your shield fast to block if you do not have your two handed weapon equipped if I remember correctly can be done if you have the shield on your arm at the time. I don't know if I clarified that.