I think you are making something wrong. In Chinese characters, there are many different kinds of strokes 横（一）、竖（丨）、撇（丿）、点（丶）、捺（㇏）、折（亅）. Directions are only one of the differences. If you only cares about its directions, you cannot write the strokes properly.
In the ancient China, people use ink brushes instead of pens or pencils, and ink brushes are more easily to express the strengths when writing (notice the difference in width of different part in a stroke).
The phenomenon you have mentioned is not the change of direction, but change of strokes instead. In 舌->舒,
横撇 turns to 横, while in 牛->物, 横 turns to 提(㇀).