I have a guess at this...
It may be similar to getting pretty ill.
Our bodies may go into a sort of shock, where our defenses are looking to utilize resources where needed most.
When we eat, a lot of resources are directed to digesting the foods and distributing nutrients.
So the first week(s) of a long hike may stun our bodies into crisis mode and actually not trigger hunger as normal to redirect needed resources say to our muscles, bones, tendons, etc as a natural defense.
When our bodies adapt, adjust or simply realize that we are going to live - the body regains a desire for more nutrients to make up for the loss and make way for new requirements or amounts our body needs.
This is just my theory.