I have noticed that every now and then.
Its as if Jerry hates it when someone other than himself is in the spotlight. I dont think that it has detracted from the show, yet.
Maybe hes a little insecure and tries to compete with the other actors on the show.
I have to agree....and while we're on the subject, I know this is a "Seinfeld" group, but I've frequently noticed the same thing on "Friends." It's as if they're just waiting for each others' jokes to finish, so they can laugh at them before beginnig their next line.