It might be that by 2008 they were already used to Bush being around, but Obama is still young and fresh in his 100 days.
It might be that Bush at some point asked them not to keep standing up as he walks in.
IMO, though, this is the hazard of making your public image to be a "regular guy you can have a beer with", as I've heard so many Americans explain why Bush seems to be such a complete and utter nitwit.