Besides being an agent , I am a mechanical engineer. It is impossible to accurately speculate as to the cause and risk these cracks portend based on the information available. I recommend you hire a structural engineer to provide an assessment. A regular "home inspector" is not qualified to make this determination. As has been stated, the cause can be as simple as normal settling, compacted soil under the slab (which may or may not be a problem in the future) or expansive soils that have not been property drained or addressed in the design. I also had a few cracks show up in my house due to some swaying during the last earth quake, but they are not a structural concern given the house design, so they just need to be patched.
If you need the name of a structural engineer, please call me.