Sanders move came after the traditional roll call had proceeded as usual. Clinton had secured a majority of the delegates votes a few moments earlier when South Dakota announced its delegation vote.
While the states went in alphabetical order, Vermont asked to be passed up to go last. Following a speech from the head of the Vermont delegation, praising Sanders for his work, Sanders stood up and moved that the rules be suspended and that Clinton be nominated by acclimation.
It had been rumored that Sanders would join the Vermont delegation for the roll call vote, but many weren't sure he'd do it until he made his way down to where the delegation was sitting while the votes were being cast.