Moles, armidillos, prairie dogs...
Sinkholes caused by excessive watering
Collapsed sewer pipe (are the soft spots in a line, or randomly spaced?)
Or, if you are particularly unlucky, one of these:
Massive Crack Opens In Earth In Texas
http://www.freerepublic.com/focus/f-news/1432848/posts
They are unevenly distributed in the front and backyards. It's really strange how quickly these appeared as I've been out hand watering for a couple of weeks and was oblivious to them, if they were there, and I don't think they were.