From the July14th blogJuly14th blog, Jeff writes:
This is still a vote-based process, unless a moderator intervenes. If the post reaches the close vote threshold (currently requires 5 close votes, with a majority of the belongs-on type), then it is migrated to the other website.
As you know, there has been a good amount of talk about moving questions by low rep users to SuperUser, specifically that the site is currently in beta, and even though it is publicly available (if you know who/how to ask) it might not be obvious to the user where his/her question has gone.
With this in mind perhaps a moderator intervened, or perhaps there was a decision to not auto migrate to SuperUser for the time being.
Apologies for not providing a real answer, but adding this color via comment would have been burdensome.