NBC only wanted to renew Medium for 13 episodes. CBS (whose production company produces the show) wanted a full season pick-up. NBC said, "No thanks" and CBS had to scramble to add Medium to its fall lineup.
CBS wasn't happy about it and publicly admonished NBC for not renewing the show, calling the decision "inexplicable".
Recently, CBS began airing reruns of the show on Tuesday nights (new episodes will air on Friday nights in the fall). Now, according to Variety, NBC is protesting, and seeking reimbursement from CBS because per licensing agreements, it has the exclusive rights to the reruns in prime-time until September.
Neither side is commenting publicly, but assuming that CBS didn't already have an agreement with NBC to air the reruns, NBC has a right to be miffed. Sure, it didn't renew the show, and sure, it wasn't going to air the reruns in the summer anyway, but if it owns the right to do so, I find it odd that CBS would take such a "screw you, NBC!" stance.
One thing that the Variety article doesn't mention, and I think likely plays in somehow both with NBC not renewing the show and CBS having to air the show is that cable network Lifetime has a syndication exclusive for Medium where it is rumored to have paid around $1.3 million per episode.
There's no doubt we're missing details about what really went on during the renewal negotiations. I'm not sure whether NBC committed to something and then reneged, or something else, but CBS seems...provoked.