If you listen carefully with the sound up on that clip you hear multiple explosive charges going off just before the guy on the phone screams "Oh God!"

The Towers didn't collapse from melted steel and some stupid 'pan cake' effect by the way. They were demolished. Lots of evidence has been collected to support that fact.

Watch this September 11th Revisited: Were explosives used?
and this Loose Change 2nd Edition Recut

Make your own conclusion.