From what I've read in the past 24 hours Dortmund contacted him in December, while he was still waiting for an offer from Bayern and he made up his mind before Christmas, they just didn't want to announce it so early and wouldn't have done it yesterday either, if it didn't leak already via the press. In his statement he said that they made him feel valued as a player and person, so I guess he's genuinely upset at Bayern's management.
There were reports that he's opened up a bar with his brother in Frankfurt (where they grew up) and wasn't so keen to play abroad anymore, which fits with this claim:
The perfect offer from abroad didn't materialize, so he rather stayed in Germany and signed for Dortmund.