You could be getting thatch from natural die off of leaves when grass goes dormant(brown in winter). I suspect it is due to dormancy. Generally speaking though a bad thatch problem is built up through poor maintenance of your grass. Allowing it to grow too tall then cutting without removing the clippings is one possibility. Another is incorrect usage of chemicals, over fertilization with too much nitrogen will cause accelerated blade growth which when combined with the first one will build up a thatch layer as well. If it is St. Augustine grass, wait until your grass is actively growing (Junish)If you do choose to rent a dethatcher machine or have a company do it for you be sure (as St Augustine is pretty sensitive ) that the machine has vertical blades that you can adjust out to 3" apart. Fertilize after you dethatch. Sharpen your mower blades and bag your clippings every other mowing to reduce the amount of clippings going on grass.
2 1/2 inch height is good for St Augustine. if you go any higher than 3, it will make it look pretty raggedy. Take a look at Walter Reeves has to say about maintaining St. Augustine, it may help you make some decisions on what to do with your grass. http://www.walterreeves.com/lawn-care/st-augustine-mowing/
The recommendations for growing heights may be a little different in Georgia, but the University of Florida Extension Service says 3.5 to 4 inches is optimal for most St. Augustine cultivars. That means setting your mower on its highest setting. You need to mow frequently enough that no more than one-third of the leaf blade is removed. Mowing too infrequently and watering improperly are the biggest reasons for a buildup of thatch.