I think there are two things; first, a grading is different from training 'cos you concetrate on different aspects: often on the names and variations of techniques and multiple attacks etc. After settling down for a while you get back in to learning REAL aikido mode where you again study concepts of aikido. That is why I think too much grading can be a bad thing.
Second; I noticed with a friend of mine that his aikido had suddenly improved over the last few weeks; and it wasn't just a small improvement it was like he had discovered something magic in his aikido. It's taken me ages to work out what had happened: he had been unemployed and as soon as he got a new job it had improved his confidence and his aikido benefitted amazingly as a result.
Therefore this new Dan grade probably still feels unconfident with his new title and it will take a while before he feels happy with it - also; improvement isn't step by step as you go up the grades. The grades are just an approximation of the ability. Most Dan grades you know probably have been Dan grades for longer than a few weeks.