There were a few reasons for the changes. First was that I wanted to better establish a foreground/middleground/background relationship and an intervening rock outcropping seemed appropriate. Second, I thought it was silly for a lighter mech to be going toe to toe with two heavier mechs at such close range. I felt it was more "realistic" (walking tanks are silly - I love them, but they are NOT realistic) for the lighter mech to have been keeping distance from the enemy mech on the right, but was then ambushed by the other enemy coming from behind the rock outcropping. Poor guy never stood a chance. I also wanted to have the closer enemy mech in a different position from the distant one. I actually chastised myself for doing that, as it was more work, but I thought it would be more visually interesting to not repeat the same pose (or, at least, similar angle), so the sides of the legs were to be shown for variation only. Finally, that corner was pretty barren with nothing there and the eye kind of aimed at it. Notice that the rock outcropping leads the eye back to the mechs. It helped create a circular eye-flow.
Hope that helps!