Attack target mechanics question

In the attached image stacks marked A and B were in battle with the stack closest to the arrowhead (call this unit D). Stacks A and B are on the same side and C and D are on the same side. Unit C was coming from the direction that the arrow indicates. When it reached stack B that was defending stack D it attacked stack A (the artillery) which was a 19 minute march away. In this case why on earth would C attack A instead of attacking B? B is the closest (at the exact same point actually) and A is really too far for it to even attack.

http://prnt.sc/dhz0rv