Firstly, there's not a migration path from Stack Overflow to DBA so if a migration was necessary it would have to be by moderator action.
I'm not a moderator on SO, but I assume that there were flags on the question suggesting that it would get better answers on DBA - moderators don't have the time to go looking for questions to migrate (especially on SO) so would only react to a flag. If that's the case then the moderator obviously agreed and migrated the question.
This could also be true of Wordpress plugin development questions or Game Development questions. Both these areas are on topic for Stack Overflow, but as dedicated sites exist for these areas questions could, and should, be migrated if they will get better answers on the other site.