Because diagonically, the front right for example can land you to back left, or otherwise screw up and land on another group. If you go with explosions first at the back you don't care where to land exactly as long as it's at the back. The melees will explode second and they can land whenever even if they screw up a bit.
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I get that, assuming no ranged are soaking at front; it's just for an optimization to reduce the effect of screw ups; if they can choose between software behavior, there's no reason not exploding the ranged first for an extra security.