According to Birkett, the news comes from general manager Martin Mayhew. It sounds like the Lions want to simultaneously keep him in Detroit, but also reduce his cap hit for the 2013 season.
That cap hit comes in at $20 million-plus. It's not that Stafford doesn't necessarily deserve that money, but the team could use the cap space, and probably figured they'd have extended him by now. They've been talking about a potential extension for some time now, but nothing solid has materialized.
Stafford had an uncharacteristically poor season in 2012. All of his numbers were significantly down from his 2011 season, in which he had 5,038 passing yards and led his team to a 10-win season. This past season, the Lions won only four games, but much of that can be contributed to injuries and lacking play from the defensive side of the ball, as well.