According to the pair's attorney Robert Schwartz, Activision's claim against them stems around an "expert's" determination that Modern Warfare 3 would have been a bigger moneymaker had they and the rest of the now-absent Infinity Ward team been present to work on it; instead of having been fired by Activision (a state that the two allegedly 'orchestrated'.)
Schwartz corroborated reports of a "Project Icebreaker" at Activision designed to secretly scour their computers for evidence that would support a firing. "Do you know who George Rose is? He was the head lawyer for [Activision]," Schwartz said.
"So George Rose goes into the office of this guy named Thomas Fenady. He's some kind of IT whiz at Activision." According to Schwartz, Rose asked Fenady, under the auspices of CEO Bobby Kotick, to "break into [West and Zampella's] computers and dig up dirt to be used to justify firing them." Schwartz added that Fenady "testified to this."
We'll have to wait to see how this insanity plays out in court; the trial was delayed to around June 1.