Having the floors above a crawl space move is a common problem. The floor shifts because of weakening floor joists, movement of the foundation, or movement of the interior supporting structure.
The foundation walls in a crawl space are much shorter and weaker than basement foundation walls. They also do not go nearly as deep into the ground which make them much more susceptible to movement from heavy moisture or lack of moisture in the soils. Since the floor system is framed directly to the foundation, movement in the foundation will cause floors to shift.
In a lot of cases the beams which support the floor along the interior of the home are held up by old concrete posts, stacked blocks, chunks of wood, or framed walls. Through time, the weight from above combined with changing moisture contents in the soil will cause these supports to shift. This movement in the supports will directly translate into the floors above.