I'm no expert, but to me it looks like a virus or fungal disease perhaps caused by too much water. Twice a week seems like too often to be watering, even outside. If it were mine I would take off all the blackened leaves, cut back on the watering to maybe once a week at the most, and see if it regrows. It's hard to tell for sure in the picture, but it looks like it's got a few pups coming out beside it. If it is making pups, it might be a good idea to pot those up by themselves, just in case your big one doesn't make it. Like I said, I'm no expert, just my thoughts :-)

Either it is rotting, or it is simply stressed by sun and heat, and isn't getting enough water, and is resorbing water from the older leaves at an accelerated rate. I suspect it is rot, but maybe not.

Watering twice a week in hot, humid GA seems a bit much too me. Are they also getting afternoon thundershowers this time of year? My potted agaves get water every couple of weeks, and there is no rainfall at all.

Unpot, remove the resorbed leaves, and examine the stem for rot. If there is soft, black stem, you can save the plant by transecting above the level of the rot. Then callous, plant on dry soil, and wait until the plant has roots before watering. If there is no rot, re-pot and enjoy the plant without the ugly leaves.