Where I work, we're only required to initial the boarding pass, but we also have to check various things on it (name, date, etc.). I would guess most TSOs have a routine worked out whereby they do things in a certain order, thus ensuring that they look at everything they're supposed to. (I know I do!) The system used by the TSOs in question probably includes checking everything off so at the end they can take a quick look at the boarding pass and immediately note whether they've missed anything. They're just being thorough, perhaps to the point of being anal (IMO)!