Plywood is assembled, at least from I can see in this video, so the sheets are arranged perpendicular to each other so that the directional durability of the grain is complimented to give an overall unidirectional strength.
When paper is milled it has a grain direction. The paper is stronger along one direction than the other. You can tell this by both looking at the paper and bending it along its lines of symmetry.
Does the manufacturing process of MDF give the resulting boards a grain direction?
If I were to glue two MDF boards together would the orientation matter for stability?