He is showing them respect (congratulate) for a great shot...as simple as that :-). He's been doing it for a long time.
I've noticed that some folks think it's 'condescending' or showing 'false appreciation', or anything the like (
), but I've never seen anything wrong with a player appreciating a good shot of his opponent, and SHOWING that appreciation.
Fortunately, most of top players do so right now, and I highly appreciate it.
It might be 'old fashioned', but I cannot see anything wrong about being courtagious, whatever the situation.