The problem is micro or surface reticulation of the film caused by the softening of the emulsion by the deveoper followed by temperature shocks in the process cycle, some films suffer more than others. Rodinal's high pH and specifically the hydroxide increase the potential problem.
Originally Posted by polyglot
The issue has been know about since the late 1920's or early 30's, it became more important with the rise of 35mm photography, I've BJPA article on reducing grain by wet mounting negatives on a glass slide for enlargement, something Ctein was still doingdecades later for the same reason.
In more recent years Kodak have done a lot of research to help minimise micro/surface reticulation. With papers it causes a mottling on the surface particulary noticeable with glossy papers, it'll happen with RC B&W and Colour papers as well as FB B&W, actually steaming removes it. With the advent of Digital minilabs and negative scanners, and also with high end scanning it became a greater issue, so Kodak altered the emulsion hardening to help eliminate the issues with their colour films making them more scanner friendly.
So some films suffer more than others EFKE will be about the worst due to having less hardening, Foma somewhere in between Ilford's films and most of Kodak's, some Fuji B&W emulsions are softer as well. Then it's a combination of softer emulsion, the developers effect on that emulsion and the care taken in the process cycle.