First, all tires develop flatspots when they are at rest. Most of the time those flat spots are small enough that you don't notice them - plus operating the tire tends to work then out.
But if you have significant differences in temperature, the elevated temps tend to make the flat spot worse.
What could also be occurring is that some of the materials are transitioning through the "glass transition temperature".
Glass transition temperature? We normally think of materials being either gas, liquid or solid (and plasma for those nit pickers!) - BUT - many materials have different states at different conditions. For example, diamonds are merely carbon that has solidified into a crystal. In the same way, some materials will change phase at some conditions of temperature and pressure.
In the case of tires, they heat up as they are operated - and when allowed to cool, they will form a flatspot - and if one of the materials has gone through a glass transition temperature, it may more readily form a flatspot - which will be worked put when the temperature raises again.
Looking at the description of the route taken, it is possible the distance to the freeway is quite different depending on which way you travel. It's the freeway speeds that are generating the heat - and if there is a significant difference in distance to and from the freeway, the tire could cool significantly before the vehicle is stopped - ergo, a flatspot generated one direction, but not the other.
