Yep that's it.
Imagine it this way - the outside of the hull is hot and expands. The floor and other interior components, however, are about 100 degs cooler and so not subject to the same expansion. In a very simplified description, if you imagine the floor on rollers but attached at one end then you'll see a gap at the other end when the exterior hull stretches. That's the effect you see when the expansion gap appears aft of the FE's panel in supersonic flight. (The floor is, by the way, not just sitting on rollers.....)
Electrical looms had a little sag between fixed points if they were attached to 'expanding' parts of the airframe.
And so on.
It's amazing there was ever any hydraulic fluid left on board if you consider the stresses and aggro involved in routing the pipes and hoses.