Personally I think he left too soon, I don't believe he wanted to hang up his helmet in 2006. Things seemed to force his hand, whether it be from personal pressures or something professional behind the scenes.
Most F1 drivers that leave the sport have a period of regret, most try to find something else to fill the void. I believe David Coulthard was feeling that this year but with his BBC Sport job had something to focus upon. He's now been linked to a DTM seat next year.
I think Schumi has had a kind of sabbatical period hoping to find something to fill his time, he didn't really cut his ties with Ferrari. Now Alonso is coming to the prancing horse, he won't take pearls of wisdom from Heir Schumacher he's far too arrogant for that. Hence Mikeys move away from the advisory role to one working with the road-cars.
Maybe he's simply bored, hasn't found himself anything else to spur his interest levels in retirement and wants another new challenge. Even if its a one year deal..he might actually want to do this.