Boeing throwing their toys out of the cot.
They're obviously seriously worried about the fixed price bid if they have to compete against EADS.
The Airbus tanker is low risk as virtually no additional work is needed, so they can bid near the bone. The Boeing tanker, on the other hand, is a paper design which, with the background of the Italian order, may cost billions in costs and penalties to builr, debug and deliver.
Even if Boeing win, the bid may have to be so low they may lose money on it for decades to come. Which makes it sensible for EADS to bid, and bid low.
Boeing source: We may not bid for KC-X