When multiple units are grouped, the initial places and positions those troops are "locked" and the troops show a strong tendency to maintain that initial position. For instance, when you have one cavalry unit fighting at the left end, and another at the right end, if you group them first and then order a move, the two cavalry units will not come together at the point you clicked, but they would be far away from each other like in the initial state they were grouped in.

Thus, you must make a habit of Rclick+dragging to issue new orders, and then group them.