Many people don't realize that during the spring and fall the climate isn't the only thing that shifts. The earth's magnetic field is also subtly distorted during these seasons causing disorientation in many animal species. Swans, as you can see from this picture, will often spend at least a couple of weeks upside down before they manage to right themselves.
Or, possibly, this swan is working just a little too hard to see what food is on the lake floor. I'll let you decide.
And a photo tip for taking pictures of swans: set your camera's meter to the center or spot setting. Swans reflect so much light that any sort of matrix or balanced metering will pretty much always overexpose the swan and burn out huge portions of the bird. This is especially true in direct sunlight.A spot setting with discount the background and consider only the bright feathers themselves.