Unfolding

Yesterday President Obama said in a speech that we are "witnessing history unfold" in Egypt, and while that may be true, I'm bothered by his syntax and I can't quite figure out why. Let's do some comparisons:

watching history unfold
witnessing history unfold
watching history unfolding
witnessing history unfolding

Watching and witnessing don't differ much in meaning, but witnessing history unfold sounds wrong to me while witnessing history unfolding sounds fine. Let's try some more:

watching a murder
witnessing a murder
watching a person murder someone else
witnessing a person murdering someone else

Why do I need to switch from murder to murdering in that last example? I need a noun phrase after witnessing: I witness a murder or I witness a person murdering, but I don't witness a person murder or witness history unfold.

But apparently our President (or his speechwriter) does. He's probably not the only one. In fact, a simple Google search shows 2.3 million hits for witnessing history unfold but 6.8 million for witnessing history unfolding. My way is more than twice as popular, but it would take a trained linguist to explain why.