We never came to a good theory. It worked fine, some dummy poured dirty tumbler water down it, it turned slow.
Current idea is to make a "french drain", vacuum or auger down to get rid of old product and replace with all fresh gravel. Local rental yard smallest auger is 4" so I'm thinking more vacuum
I remember your problem. I even remember the dummy with the dirty water......
I did have a problem once with air conditioning condensate going into a hole in the concrete that slowly plugged up. I think that was calcium leaching out of the gravel. I just drilled a bigger hole and jiggled the gravel. It worked till we sold the house..
The problem here is that I fill a 5 gallon pail daily. I'd bet my gravel couldn't take it. Someday I might run a plastic line out through the wall into the yard drain which goes to my pond. That would never plug up. I need to add that to my project list.