Determining if a foundation is moving or shifting?
Cracks are caused by 1) changing climate conditions, 2) soil movement 

1) Where you live, I doubt the climate changes much from season to season. You do not have a freeze/thaw issue. 

2) Soil movement can be caused by adjacent property movement, expansive soils or tidal/moisture fluctuations. 

Look around your property and see if a neighbor has been moving dirt or if the city just installed a new sewer/water line in your area. (Even a new fence can cause problems if an auger was used and hit a tree root, pipe, etc.)

Florida is known for having soil problems and often the ground will swallow whole houses. I doubt you live in a soil area that has “decaying” soil like Florida. However, if the developer cut down trees before building your house, the stumps could have rotted and left large holes in the ground that allows sluffing and ground movement. 

If you live in a high water table area, tidal action or seasonal rains can cause ground movement. 

However, the most likely cause is expansive soils. That is to say, when your dirt gets wet and when it dries out, it expands or contracts. The amount of movement is directly related to the amount of moisture in the ground. 

If you’re concerned, you can keep a journal noting season, rainfall, and what changes, (i.e.: doors rub, cracks open up, etc. ) 

if problems are only in the southwest corner of your house, that would be good to know when it comes time to fix it.