Typical causes include expansive soils combined with seasonal moisture changes from summer to winter combined with shallow foundations and/or from past seismic activity - all of which are very common in the Bay Area. For houses before 1950 (on average), there is generally no re-bar (reinforcing bar (steel) to help reduce cracking.

The horizontal cracking is generally due to rusting of the rebar (reinforcing bar - steel) within the foundation which is a form of 'spalling'. The rust causes the steel to expand and is a strong enough force to cause the concrete to crack. This is sometimes times most noticeable where the rebar was placed closer to the forms & in some instances the foundation is sitting on saturated soils and the water wicks its way up through the foundation over time

Diagonal cracks are generally from differential settlement of the foundation/structure.