I don't know why their base malts produce this effect. I just know that its true. I don't know that the darker malts have that effect, but their limited percentage in most grists probably make that feature inconsequential.
I've hypothesized that this is actually a good thing for most brewers since most of the county's water supplies are alkaline (see the alkalinity map that I posted on the Water Knowledge page at the Bru'n Water website). So having a base malt that helps combat that alkaline water is likely to improve beers in which the brewer hasn't learned the fine points of mash pH adjustment. So, its a win-win for the brewer and the maltster. But for brewers that use a program like Bru'n Water, they need to know that Rahr malts are likely to drive their mash pH down a bit more than expected. That isn't a bad thing, but it is something that the brewer should know about beforehand. Bru'n Water is all about predicting and creating a desirable mash pH and beer outcome!