The reason is that Atlanta is greedy for water. Georgia want to grab a piece of the Tennessee River so they can start sucking water out of it to feed their drought stricken lawns.
In 1818 the state of Georgia hired a mathematics teacher named James Camak to draw the official boundary between Georgia and Tennessee. Georgian politicians, being the same cheap-ass people they are today, refused to let Camak buy the equipment he needed to do the job properly. He had to make do with a Navy surplus sextant rather than a state-of-the-art Zenith telescope. With cheap equipment the best Camak could do was a reasonable estimate of the border. He managed to draw a straight line and only missed the 35th parallel by less than a mile.
|There is even a monument.|
Everybody was perfectly satisfied. Georgia had a gold rush, drove all the Native Americans out of the state, hosted a civil war, and nobody cared the state's northern border was off by a hair. It wasn't until the 1890's when Georgia politicians, fresh from passing Jim Crow Laws to re-enslave their black citizens, thought about snatching up some prime Tennessee real estate.
For the next century, Georgians would periodically bring up the border dispute but mostly because Georgians tend to be royal assholes. They didn't really want or need the land, they just wanted to fuck Tennessee.
Now they want the land, or rather the water, badly. Georgian politicians may condemn climate change science as Communist propaganda but they also believe the scientists when they say climate change will mean permanent drought conditions for Georgia. They want a tiny piece of the Tennessee River so they can claim a huge amount of the river's water.
Of course, Georgia could negotiate regional water issues like Western states have done for decades, like responsible adults. But that's not the Georgian Way. Georgians prefer to resolve disputes by shooting your dog first. Georgia wants the water and they want to fuck Tennessee (whose land they will take) and fuck Alabama (the water they take would otherwise flow into drought stricken Alabama) than negotiate like wusses.