I've never experienced that kind of deformation that was that noticeable breaking down boxes...but then again 175 cuts across the grain is quite a bit and I can't recall ever cutting that much at one time without a touch up in between.
At first blush I was thinking along the same lines as Matt -- that is, brittle stressed factory edge.
But since that's ruled out...do you think you might have had a bit of a burr or wire edge that wasn't fully erased with the MicroBeveling?
Then there's the composition of the cardboard itself, which can vary widely. A lot of cardboard is recycled and contains a lot of grit / grime.
Plus, what you have on your lap looks pretty thick and heavy duty in it's own right, grit notwithstanding.
So....IMHO, all things considered-- overall not that bad.
And what I think is even more important to consider here is: How LONG will it take you to erase that damage on the SharpMaker?
Because, after all, we've all been where you're at now at one time or another.