This is mottling.

I've seen it on my own film, which I processed myself, after the film sat in the film holder for too long. My hypothesis is that it's caused by condensation due to temperature changes; liquid water on the emulsion of the film can dissolve sensitizing dyes and redistribute them unevenly, making parts of the film less sensitive to light, and parts more, or changing spectral sensitivity locally.

The main way I'm aware of to avoid it is to load film holders shortly before you'll expose the film, and unload them (into a suitable light-tight holding container) as soon as practical after exposure (and then process them as soon as practical after that). Sadly, this can also happen to film still in the box, if it's not airtight and is subject to high humidity and/or multiple temperature cycles (for instance, being put into and taken out of a refrigerator multiple times).

This means the rule for storing film should be "Keep it cold until you're ready to use it, let it warm to room temperature before you open it, then store it at room temperature thereafter."