GamesIndustry.biz understands that Harrison grew frustrated when Mattrick's departure and Spencer's subsequent promotion did not result in him moving up the corporate ladder, issuing the ultimatum to Microsoft that he would leave if not offered a new position. However, it seems that no offer was forthcoming, and Harrison has made good on his promise.
When asked to comment on the matter, various Microsoft employees essentially replied with a "no comment" type response. Here is the responses given by Phil Spencer from the Xbox division.
"I don't, no. I'm sorry, I don't think I can say anything about that," remarked Phil Spencer.
When ID@Xbox's Chris Charla was asked about it, a Microsoft spokesperson stepped in to say that Charla wasn't the person they should be talking to about staffing issues. They also then said that the company does not comment on rumor or speculation.
Prior to working at Microsoft, Harrison spent some time (16 years) working at Sony.