Can one moderator just make a unilateral decision and action a migration? Or were there multiple votes involved?
Regular close voters can vote to close a question, or choose from a limited set of sites to migrate it to. Code Review is not an available migration path on Stack Overflow, however, so only a moderator may migrate a question to that site. A moderator's close/migrate vote is always binding (i.e. it overrides all others).
You can find out who performed the migration by scrolling to the bottom of your question where it was moved to, and clicking the origin site name:
migrated from stackoverflow.com
Which brings you to the original page, where you can see at the bottom who migrated it:
migrated to codereview.stackexchange.com by Luchian Grigore, casperOne♦
That said, a migration can be rejected; this involves closing the question on the destination site, which unlocks it on the originating site allowing it to be reopened if appropriate. However this depends on what happens to the question on the destination site.