Vinyl lined pools have steel galvanized pool walls 98% of the time. These steel pool walls are supported by “A” frames or “X” bracing at the back of the panels to support them and keep the straight. These types of pools are also designed to always be full of water which helps support the walls outward. The steel pool walls are also supported by a concrete footing that locks the walls in place and forms one continuous unit instead of having individual panel segments making everything stronger. Problems with these pool walls moving are most of the time related to water loss from the pool. This causes frost heaving and the pool walls to creep inwards over time and also destroys concrete and older style plastic coping. A lot of pools were built without concrete footing and these are a lot more prone to wall movement from frost. Once the steel pool walls push in, there is no way to push them back out without damage. The backfill from behind the walls sinks down and prevents them from being able to be pushed back. The only option is to remove your concrete pool deck, excavate the fill, and pull the walls back. This is an extensive project and financial commitment is needed.
Tree’s close to the pool that suck up a lot of ground moisture can also cause your pool to become out of level. When you have a large tree close to your pool that removes a lot of moisture from the soil, the ground will shrink causing your pool to sink and shift. The reverse can happen if that tree is cut down and the ground then becomes saturated.