There was this brief period when Picard was captured by the Cardassians while on a secret mission, and instead of making Riker acting Captain they brought in this Jellico guy, because Starfleet didn't trust Riker's judgement because he had turned down a command of his own to remain as First Officer on the *USS Enterprise*. Jellico absolutely despised Riker, thought of him as a nest-feathering schemer, so Jellico was obviously picked by a fairly anti-Riker faction down at Starfleet HQ.