As we all know, the destruct sequence on the original Enterprise required voiceprint and code verification from the top three officers serving at the time.
In "Let That Be Your Last Battlefield", the original episode where the sequence was introduced, those officers were Kirk, Spock and Scotty (who were established as the top three command officers; Spock served as Captain when Kirk was away; in the absence of both, Scotty took over.)
So we get to the destruct sequence in TSFS. Kirk kept the first code; Spock, of course, was otherwise occupied, so Scotty moved in to the second slot. When the sequence was initiated, why did they bring in Chekov for the third code? Why not Sulu, who fell right behind Scotty in the Enterprise chain of command? Had Chekov become senior to Sulu by virtue of the fact that he'd been a First Officer in his own right until just prior to the events of the movie?