There seems to be some bugs in movements of army. When you call your artilery, they stuck sometimes on some rock or wood, or in this case, tank and cannon seem to come into some kind of black hole where they start to turn around and cant move foward and backward on this bridge. Hope it will be fixed in next patch. Patch is 1.81.2
http://img151.imageshack.us/img151/9439/53999744.jpg