Wow!!!
Very unusual for that many factory rounds out a box to be bad.
Sounds like powder contamination, possibly somewhere during production, or somewhere during storage, or transportation to vendors. Does the box have an oily residue on it like something soaked into the cardboard or do the unfired rounds feel oily?
After you purchased them, was there any potential for them to have come into contact with any type of penetrating oil like B'Laster, Kroil or something like that? I have known one guy at my old club that had a bad batch of factory cartridges and they did the same type thing you describe. He traced it back to the spillage of a gallon jug of PB B'Laster penetrating oil in his loading/cleaning room.
I'm very glad you discovered the problem before it could have become a bigger problem (like in a SD situation).
Someone on here (Richard E., I think) posted once before that they like to buy SD ammo several boxes at a time so that the lot numbers match. I try to do the same, and usually buy at least four boxes at a time. I also test a number of rounds out of a box before I carry them in a SD gun.