Moles?
Gophers?
I'd also guess moles or other burrowing animals.
Find any cattle mutilations in the area?
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
Is there any pattern ? Is there any moisture at all ? Could it be your sewer tiles from the house to the main sewer are broken ?
Several years ago my lawn went to hell when my sprinkler system broke down. After I fixed it I watered and watered to bring it back to no avail. What finally worked was a mixture of fertilizer and insect killer. The insect killer is as important as the fertilizer. Check with your local supplier to find out what type of fertilizer/insect killer works in your area. I use a fertilizer that both fertilizes and kills weeds--works wonders.
As to the watering: I was told to water from about 4AM to 10AM. This allows for the water to soak in and let the sun evaporate anything left on the surface. This prevents mold from taking hold and growing. I was told that mold will hit if you water a hot lawn, so let it cool down overnight and then hit it with water in the cool morning hours.
So far all those tips keep my lawn nice and green through the summer.
Was the ground dry? Overwatering could have washed away the soil, perhaps?
Moles.
Definitely moles. Same thing happens with my lawn. If i don't water then they go off looking for softer and more earthworm ready dirt to hunt in. Once you water you make it more viable for grubs/earthworms and stuff and the moles come to feast.
About those people telling you it's moles and you need to get rid of them:
HA!
If it's moles, then you no longer own your yard period.
Grubs and ants would be attracted to your lawn because of the moisture. Use diazanon, an insecticide. One brand name is "Sevin" Use it liberally and often. Kill the moles food supply and it will discourage them. - They will leave if and when they get damn good and ready and not one minute before.
If moles are present, they are present year round too. They go deeper in very hot and dry weather and in the winter time, hence no tell tale burrows above ground.
Could be Voles too.