I'm pretty much convinced that the SEC absolutely wants to expand to an even number as soon as possible but also that at least a good number of SEC schools was to look east before adding Missouri. If the SEC expands to fourteen, they will grab an ACC school. If they go to sixteen, they will add Missouri and two ACC schools.
Missouri is in a bit of a bind at the moment as they likely have a very small window before they will need to affirm their B-12 membership (and commit to the six year TV rights ceding) so if they cannot land in the SEC very soon (I'm guessing they may not even have until the end of this month), they will be stuck in the B-12 for many years.