There is no way in hell that looks like typical HC-110, first of all. That is pure hogwash. 35mm HP5 in HC has less grain.
My first thought in any grain question, responding blindly, would be overexposure.
Second thought would be underexposure that you have "printed" up.
Third thought would be old and/or heat-damaged film.
Fourth thought would be that you somehow "shocked" your emulsion during processing. For instance, with a rapid and major temperature change.
In this case, another thought would be that it might be caused by the inadequate fixing leaving silver in the emulsion that should have been removed, thus lowering the contrast of your neg and forcing you to heavily manipulate in the computer, thus enhancing the grain. And/or, perhaps the residual silver somehow makes grain more apparent.
I would tend to go with the exposure explanation, but would have to see the neg to make sure. It looks like a thin neg that has been extremely lightened in the computer or printed way back in a darkroom.
I just read your last post about how the film is not yet clear. Well, there you go...Take care of that first, then judge. This is similar to an overexposed and extremely low-contrast neg as far as your computer knows, so everything you or your scanning software did to make the pic look somewhat "normal" just exaggerated the grain. Why didn't you just fix it again, especially since the incomplete clearing is plainly visible?