Notice how the horizons don’t align on this warped tub – the rear right corner is sinking
The ground under gravel, paving tiles or brick pads can swell or sink as time goes by, and the hot tub can slowly twist out of shape as this happens. Sometimes people shim their hot tub to make it level, but by doing so, it creates a hollow under the tub, forcing the tub to sag in the middle as it settles. And other times, a poorly made shell can twist and droop for no apparent reason at all, due to physics beyond our comprehension.
This gradual incremental warp doesn’t usually damage the shell if happens over many years, but as the shell ages, it becomes less resilient and cannot be bent back into the original shape without cracking. If a used warped hot tub is placed on a proper flat pad and filled with water, it cannot immediately undo years’ worth of twisting, and the strain can crack the weakened shell.
Likewise, if you put a perfectly square used hot tub onto a warped pad, the same kind of damage can result. The pad doesn’t have to be exactly level – no problem if one side of the hot tub has a higher water table than the opposite side. But do make sure the pad is completely flat, so that every square inch is touching the pad for equal distribution of weight.