While environmental factors may be contributing, think first about acute pest problems such as the southern chinch bug (only on St. Augustinegrass, and mostly in the summer), brown patch (winter time), and human-caused stresses. 

For example, if the turf is brown or yellow in sharp-sided polygons, it may have suffered a human-caused stress such as vehicular traffic, smothering under a tarp, a buried septic tank, etc.  If the damage is a blotchy circle, it is very possibly due to an insect or a fungus.  Are other unrelated plants in the area showing the same symptoms?  If so, you might be looking at chemical burn or salt damage.