It's interesting that the v-roll doesn't continue rolling so that we can get a better context of the action.
It looks to me like Bush is just acknowledging Clinton's presence or trying to get his attention, or letting him know that he's passing behind him. Bush has always been known as a "hands-on" guy, so it's probably just a normal action for him.
I notice that Clinton turns toward Bush at that moment, so it seems that Bush was also speaking to Clinton, so I go with the idea that he was trying to get his attention.