Well, it hasn't happened again last turn, which is good ...
I have a theory that might explain it. This happened around Washington DC where there is a very high density of stacks, especially on the Union side. (For some reason the latest version displays all stacks in a region separately, plastering the map with stack icons. It looks scary!)
Suppose I had inadvertently targetted an enemy stack rather than the region I wanted to attack. Then if the stack in the region retreated, my attacking army would follow ... into Washington, or to York, PA, wherever the other chaps went.

[color="Gray"]"These Savages may indeed be a formidable Enemy to your raw American Militia, but, upon the King's regular & disciplined Troops, Sir, it is impossible they should make any Impression." -- General Edward Braddock[/color]
Colonial Campaigns Club (supports BoA and WiA)
[color="Gray"]"... and keep moving on." -- General U.S. Grant[/color]
American Civil War Game Club (supports AACW)