From what I've read, there are two things going on here:

One, IW had the right, per their contract, to do something other than CoD as their next game, and they intended to. This freaked the fuck out of Activision because they want a sequel NOW to pump up the profits and there is no way they would be willing to let IW do something else. So they fired them on some trumped up pretenses and farmed the franchise out ASAP to get the max amount of money out of it.

Two, both of the guys fired were up from some kind of bonus soon, so way pay them when you can fire them for something else? By mostly, it's number one. They niavely assumed Activision would honor the contract and let them do a new game. After seeing the sales numbers for MW, there is no way that was going to happen. So now the franchise will be whored out for max profit and screw the quality.