I wonder how much of that is peer pressure (since its not a secret ballot is it?) and also a perversion of choice though. IE in some ballots Buttigieg would have been over 15% so people could vote for him, in others he'd have been under so they couldn't - ditto for Biden etc - but Sanders would have been almost universally available to vote for in the second round.
If someone is in the second round all the time while the other candidates are in it only half the time, it seems likely that the one there all the time would naturally get more votes - whereas if the choice had been just the same two in all wards it might have been different.