Counterfeiters like to pass one fake bills at a time in a pile of real ones, to make it much less likely to be discovered, and to be able to credibly claim that they had no idea about the bill.
As we know, the Bushies used North Korea as part of the "Axis of Evil" to keep the Axis from being solely a Middle-Eastern construct. What if firing the other attorneys was the same strategy?
We know that the Bush team has a fairly limited playbook, albeit one that was successful for years. We also know now that Lam was closing in on Cheney. It would seem, then, that firing the other USAs at the same time as Lam would give the operation cover. They had no reason when discussing Lam that they were in any danger of losing Congress, and they still felt (maybe still do) that they could get away with anything so long as it looked good enough for the media.
If they had target both Lam and Iglesias, the theory still holds; the idea was for the firings to not look retaliatory or an obstruction of justice. Sure, they had problems with several other USAs -- this gave them the perfect opportunity to throw them all under the bus. But if Lam was about to investigate Cheney, then Lam was the main target, and getting rid of a few others, or even all 93, was really just a cover.