Maybe it leaked a little water into it. Then in the winter it froze and expanded. Think polar vortex last winter. Then spring came, and them warmer weather dried things up. But the damage is done. But you don't see any wetness now.
Being from NH, we see that sort of stuff often...when spring rolls around. Not all that likely in Georgia. But that polar vortex could have changed that this year. We camped a week in Hiltonhead for a week of less than 10 degree temps.
So the real problem just may be poor sealing. But the wood might need replacing too.