Here is an interesting little experiment you can do at home with the kids. Take some plastic food wrap, and tear off a rectangle section thats around 5cm (2″) wide and as long as the box. Try to get an even tear.
Then cut off another section that is a square, rotate it 90 degrees and place it on the same tear strip. Then tear it to the same size as the first bit. Keep note which is which.
You now have two strips of plastic wrap of roughly the same size but from different orientations on the role.
Now take one of the strips and hold onto the ends and gently try to stretch it apart. If you do it slowly it will stretch and then at some point break. Do the same for the other strip.
What you will notice is that one strip of plastic wrap has stretched more than twice its length while while the other just breaks after very little extension.
So why do these two strips of different orientations which look the same have such different properties? Well the following is my best guess.
The plastic wrap I have started out as LDPE nurdles, which are melted and then extruded into a large balloon like structure. During the extrusion process the longer chained polymers all become aligned in one direction. Just as a coiled garden hose will straighten if you drag it behind you, the friction between the long polymers and the extruder die will cause them to align in one direction.
So the strip of plastic wrap cut in the direction that the roll of plastic unwinds has all the polymer chains aligned in one direction. This makes it stronger, but at the expense of ductility. The strip cut in the direction of the box however has far fewer bonds (branching between chains and secondary bonds) and so is much weaker. When its stretched through, these branched bonds are able to break and reform at new locations, thus allowing the plastic to stretch by incredible amounts.
Another good example is wood. The wood is much stronger with the grain, than against it for similar reasons.
How does all this relate to repraps? I suspect the same effect takes place in extruded filaments and so may play a part in warping. If not, then its just an interesting side note. Something to think about all the same.
Here is a short video on the manufacturing process if your interested.