Is my pool cracking due to sitting empty during freeze-thaw cycles?
I used to install pools with Zonolite / Portland cement mixture, later years Gunnite. The shells were usually only ~2" thick. That area had a high water table and if someone drained the pool the bottom shell would break and water would enter between the liner and the Zonolite/ Gunnite mixtures. If there were leaks in the liners they usually found a crack and could also cause the problem your pool has. We would drain the pool in the late summer pull back the liner and fill the depressions and cracks with hydraulic cement. If water gets between the liner and bottom be very careful filling the pool because this can rip the bottom drain out.
I would suspect the leaks caused the problem. Even if this is a thicker bottom once the earth and sand are eroded the bottom has no strength.