Normally, sunset moves vertically, from up to down in the sky. But in this photo, it appears to be moving horizontally, from left to right. It might be an instant of a phenomenon related to scattering of light.
If it is important, the location is Isla Mujeres, Quintana Roo, México according to the original poster.
How can we explain this phenomenon?
I did some research and it might be related to a passing jet stream disturbance that occurs in middle latitudes where low, thick stratus clouds are moving west to east and it happens to be happening during the sunset. Of course, the location of the observer is important as he is far enough not to see the clear strip near the horizon. At the dark side, the clouds are covering enough area to block the sunlight for the observer.
I've tried to draw in Paint also as below:
Please correct me if I'm wrong.