In the example in the OP, there's basically a gap in the clouds that is letting in one ray of the setting sun that goes all the way across the sky. A lucky arrangement of clouds and timing.
Anticrepuscular rays are similar to crepuscular rays, but seen opposite the sun in the sky. Anticrepuscular rays are near-parallel, but appear to converge at the antisolar point because of linear perspective.[1] Anticrepuscular rays are most frequently visible near sunrise or sunset. Crepuscular rays are usually much brighter than anticrepuscular rays. This is because for crepuscular rays, seen on the same side of the sky as the sun, the atmospheric light scattering and making them visible is taking place at small angles (see Mie theory).
Although anticrepuscular rays appear to converge onto a point opposite the sun, the convergence is actually an illusion. The rays are in fact (almost) parallel, and the apparent convergence is to the vanishing point at infinity.[2]
I can understand (and most probably have seen) crepuscular rays but the anti ones scrambles my brain as they converge opposite the source
Ah, I get it now. The fact they "seem" brighter as they narrow, giving the impression the source is opposite the sun is also explained. I think that's where my thoughts stalled.
Explanation: What's happening over that castle? While waiting for the Moon to rise last month in Thurso, Scotland, amateur astrophotographer Stewart Watt took a three minute exposure of the background stars. The above image was the surprising result. Patchy clouds in front of the rising moon created crepuscular rays streaming across the night sky in spectacular fashion. In the foreground is a stone tower from Thurso Castle, a 12th century fortress augmented in the 17th century. Above thecrepuscular moon rays are stars, many from the constellation of the Lion (Leo). Visible to the right of the tower is the planet Saturn.