It's the same with senior government ministers, isn't it?
I vaguely remember outrage over Blair and Brown travelling separately to a conference - a waste of government resources etc - but the more rational commentators said it was more to do with continuity planning in case of a crash.
But this is a couple of decades ago, so I may be misremembering. Anyway, it stuck in my head that senior members of the government travel separately to avoid the risk of half the Cabinet being wiped out in one stroke.