I've thought a lot about it and here are the reasons I think it happens:
There are a lot of hunters in any area. Each supplier (garda, brinks, dunbar, loomis, etc) has a lot of hunters. Some probably dump right back into their own supply and others dump into other supplies. I've always ordered garda in my area. If I'm doing 20+ boxes per week I am at the least moving 20 boxes of dumps out of my supply and into someone elses supply. I know there are a ton of halves in circulation, but I think over time that can have an impact on some of the people that are hunting from the other suppliers. If I'm dumping 28 boxes a week, every week into dunbar, then those dunbar hunters are going to have that much harder of a time finding something good and that much more likely to give up hunting or slow down hunting until it picks back up. That can in turn also lessen the amount of dumps I have to hunt through every week in two ways: 1 by physically hunting dumps and moving them out of my supply and 2. potentially lowering the number of people dumping into my supply. I've seen what I and probably other hunters have done to dunbar and brinks in my area first hand. The boxes literally are not worth looking through even if you don't have to order them.
The other important factor with volume hunting is that halves seem to be really streaky. There can be a great collection dump that is in a very small number of boxes with a lot of skunky boxes surrounding it. The more boxes you are hunting each week makes it that much more likely that you are going to get one of those awesome boxes. I've had days where I have 5 boxes with 4 giving up a total of 1 or 2 40% and the last one giving up 20+ silvers. If I was still hunting low volume I could have missed the good boxes and had a meager day.
Finally, by moving a massive volume of coins over time in an unnatural pattern from one supplier to another makes it more likely that your pickup supplier is going to run out of coins to supply. They are going to have to get them somewhere. I'm not going to guess where that "somewhere" is because I'm not in the cash/coin management industry, but I do know that they are likely to be new to me coins.