i had to research this for the rapid fire news on tonight's podcast, so i know a bit about this..
Activision confirmed the firing of the CEO and chief tech officer due to "insubordinance", basically these guys weren't obeying authority or something.
But on the flipside of the argument, Vince Zampella said in an interview a couple of months ago that Activision never wanted the CoD series to become modern, that they wanted the games to stay rooted in WW2-type thing. Then CoD 4 went on to shift 14 million units.
my guess would be that those 2 guys and the Activision team locked horns over future decisions, and the people at the top swung their hammers of authority.