It's just arm chair psychology, he agreed the new deal behind the scenes in mid-August and still had a good start to the season against Liverpool in the CS followed by Fulham. His performance declined as the team got worse following the October international break.Not sure the new contract has that much of an effect tbh.
Auba isn’t a young player signing that first big deal, he was already wealthy before this deal I think he just doesn’t enjoy his role in the team and the results we are getting and at this level even a 5% drop in motivation would be noticeable.