I've been all over that fort since 1999 and I never noticed this little detail until a few nights ago while I was picking out stuff for Pic of the Day.
If you look at the right-hand wall where it meets the back wall, you can see a sizable lean-out at the top. This is the same room as in the POTD for Feb. 21, looking at the opposite end. (click to make it larger)
I'd remembered from reading the Historic Structure Report for Pickens that engineers had trouble with the south face of Bastion B. The rooms of the three bastions on the west front--B, C and D--are built differently from nearly all the other casemates (rooms) in the fort in that those big arches are turned directly off of the outer walls instead of standing independently of them. There's a lot more stress on this wall, then, than there is on the one on the other side of that small doorway.
The fort was built on sand. All of Santa Rosa Island is a glorified sand bar 40 miles long, less than a mile wide. The only thing that holds it all together is the vegetation, especially sea oats. Sand doesn't compress--not like clay. The grains are still able to slip no matter how tightly they're packed. When the engineers started building the fort, they dug up tons of sand, leveled and tamped the area, laid down a "grillage" (3-inch thick birch planks laid up in two layers as a footing for the foundations), then started with the masonry itself. The island's only a few feet above sea level (depending on which dune you're on), so the fort's foundations are under water, which keeps the wood from rotting.
The wooden grillage helps to spread out the weight of all that masonry, but there's bound to be some settling. That one wall likely started leaning out as soon as the weight of the overhead arch came to bear on it. If you look at the small doorway, you can see several concentric arches where Engineer Major Chase tried to add support, almost plugging the opening with solid brick. Between this and adding 3 feet of brick to the outside of the south wall, the motion of the south face was arrested.