By the time he relieved Riker, he was well within his right to be considering other options for the sake of the mission
Picard himself went through a few chief engineers before settling on the one he wanted.
Or was it Riker too jealous that he was passed over for command of Enterprise? I sometimes think he was waiting for Picard to die/retire to get the Enterprise for himself.
That's something I hadn't considered, that Riker problem with Jellico stemmed to a large degree from Riker not being given command of the Enterprise in Picard absence.
Riker had to know that his previously upwardly mobile career was stalling, here he was the bright eyed boy who was offered one command after another, but when the command that he really wanted opened up, he was passed over in favor of someone else..