Likely their tanks were pressed full coming out of the yard. As has been mentioned, it isn't just liquid ballast, it's also the speed at which that ballast can move in the tank. One of the requirements of a stability test, or inclining experiment is to perform the experiment light ship, or if you do have anything in the tanks, calculate for it.
As fuel/water burns out, GM gets smaller and smaller, leading to a reduced righting arm, which seems exactly to be the case here. When newly launched, the ship felt fine, but after the first trip, the ship took on a list which switched from side to side as free surface area of the tanks took effect.
I work on Destroyers, which ballast the main fuel tanks with seawater for exactly this reason. If burnout played a role, the ship would never be stable. No more than 2 tank pairs may be empty at a time if the ship is pierside, never underway.