Attachment 418I've been noodling on this for a while and think I can explain it.
The usual explanations of single slit diffraction show it in 2 dimensions, as in these web sites.
Fraunhofer Single Slit Diffraction
Exercise, Single-Slit Diffraction
In the real 3-dimensional case, the waves are actually concentric spheres rather than circles. In order to be able to see the light on the screen, the beam has to have some thickness, and the light from the upper part of the slit has to fall in the same area as the light from the bottom part. If the slit is vertical, at right angles to the beam of light, and the light falls on a vertical screen, then the light from the top of the beam reinfoces the light at the bottom of the beam, in the center of the spot of light.
Now what happens when you tilt the slit back, at about a 45 degree angle? The light from the top of the beam hits the slit at a different point in its phase than the light at the bottom of the beam. This means that the light will not reinforce at a point directly behind the slit. Instead, the light from the top of the beam will be in phase with the light from the bottom at a distance as shown in the attached sketch. Each successive maximum will be a little higher than the previous maximum. The horizontal dispersion will be the same as when the slit was vertical. Adding these two effects together gives you a curve that goes out and up, hence a semicircle.