Stasior's reasons for leaving Apple aren't clear at this point, but Giannandrea's hiring is a likely guess. Since his hiring last summer, Giannandrea became very involved with the development of Siri and was eventually promoted to a senior vice president, reporting directly to Apple CEO Tim Cook.
Although Stasior was in charge of Apple's Siri team since the assistant's initial launch, he won't be working on Cortana at Microsoft. The company behind Windows is lagging far behind the competition, as far as AI assistants are concerned, with Cortana being a distant fourth to Google's, Amazon's, and Apple's offerings. However, Stasior's talents apparently won't be used to better Cortana, but rather in Microsoft's broader AI efforts. The Information reports that people familiar with the matter have said that the former Siri chief will head a special AI group at Microsoft.