My beautiful St. Augustine grass is dying in patches.

what is your location

Where do you live? It could just being going dormant through the winter months? What has the weather been like lately?

Your lawn woes could be from a number of things from insects to fungus to poor nutrition, Lelpdq. See if you don't find this informative: http://neilsperry.com/notes/2017/02/st-augustine-diagnostics/

Crop circles, maybe? Then I thought maybe a dog urinated on the grass but even the biggest dog can't pee enough to clear a 10 foot circle. So I thought maybe I should look it up. Seems it might be a fungus, and the circle shape could be the way its been spreading: http://homeguides.sfgate.com/treat-fungus-st-augustine-grass-51800.html

Do a Google search on problems with this type of grass. Narrow it down to your planting zone: https://www.google.com/search?q=problems+with+st+augustine+grass&oq=problems+with+st+a&aqs=chrome.1.69i57j0l5.12367j0j7&sourceid=chrome&ie=UTF-8

Planting zones: https://www.google.com/search?q=planting+zone+map&oq=planting+zone&aqs=chrome.3.69i57j0l2j69i59j35i39j0.7905j0j9&sourceid=chrome&ie=UTF-8

Could it be a fungus called Red Thread?

If you live anywhere near Florida chances are it could be one of 3 problems. Solve by checking the easiest first - Southern Chinch bug (Type in - UF EENY-226. Fungal - Large Patch - UF SS-PLP-5 and - UF SS-PLP-11. Then St Augustine Grass - UF ENH-5 where you will learn that the grass needs fertilizing, watering, mowing at the correct height and sunlight. An unhealthy grass can lead to any of the above conditions to exist and if you have an abundance of trees in the yard that shade the lawn from the sun the grass will never grow to your satisfaction. Good luck!