Over time a hot tub can slowly bend out of shape if it doesn’t sit on a perfectly level solid pad.
Warp is often unnoticeable if you aren’t specifically looking for it, until it is relocated onto a flat surface, where it can be rocked corner to corner. When the tub is filled, it will quickly be forced to bend back in shape and the shell may crack from the sudden stress.
-Take the cover off the tub, then lower your eyes to the level where the front of the shell sill is at the same level as the back sill, and check to see if the horizons align. Do this side to side and corner to corner to check for lift/curl/twist