Evidence of A Pyroclastic LIKE Flow of Debris:

• The flow of concrete (and other material) is visually much like a Pyroclastic flow.
• The behaviour of the expanding dust clouds matches that of a Pyroclastic flow.
• The dust cloud spread over the Hudson river near the WTC Complex (indicating a large amount of energy.)
• The flow of debris was not Pyroclastic, however it was Pyroclastic-like (due to the lack of high temperatures.)
• The flow of pulverized concrete (and other materials) expanded as much as several times (approx.) as the size of each of the WTC Towers.
• These Pyroclastic-like debris flows were caused by the detonation of Nuclear Devices.

What Is A Pyroclastic Flow?

"Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from their source vents at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both, or from the collapse of vertical eruption columns of ash and larger rock fragments. Pyroclastic flows may also result from a laterally directed explosion, or the fall of hot rock debris from a dome or thick lava flow."

View Cached Source
U.S. Geological Survey
Pyroclastic Flows and Pyroclastic Surges
SOURCE_URL: http://vulcan.wr.usgs.gov/Glossary/PyroFlows/description_pyro_flows.html

The above map shows how far the dust cloud from both the WTC Towers expanded (to be accurate the map doesn't show what flowed across part of the Hudson River.)

View Cached Source
U.S. Environmental Protection Agency
Pictures from the World Trade Center Response
SOURCE_URL: http://www.epa.gov/wtc/pictures.html

There are 3 things known to cause dense fluid motions

1. Turbidity currents / Density currents

A turbidity current or density current is a current of rapidly moving, sediment-laden water moving down a slope through air, water, or another fluid. The current moves because it has a higher density and turbidity than the fluid through which it flows.
The term "turbidity current" is most commonly used to describe underwater currents in lakes and oceans, which are usually triggered by earthquakes or slumping.
In such cases, high-speed sediment-laden water flows down the slope under the clearer water, causing a great deal of erosion and subsequent sedimentation in features classified as turbidites.
Turbidity currents are characteristic of areas where there is seismic instability and an underwater slope, especially submarine trench slopes of convergent plate margins
and continental slopes and submarine canyons of passive margins.
SOURCE_URL: http://en.wikipedia.org/wiki/Turbidity_current

2. Volcanic Eruptions

Pyroclastic flows are a common and devastating result of some volcanic eruptions.
They are fast-moving fluidized bodies of hot gas, ash and rock (collectively known as tephra) which can travel away from the vent at up to 150 km/h. The gas is usually at a temperature of 100-800 degrees Celsius.
The flows normally hug the ground and travel downhill under gravity, their speed depending upon the gradient of the slope and the size of the flow.
SOURCE_URL: http://en.wikipedia.org/wiki/Pyroclastic_flow

3. Detonation of Nuclear Weapons

Below comparisons are made between the collapses of the WTC Towers and Pyroclastic Flows.

WTC vs Pyroclastic Flow:

Comparison #2:

Final Comparison - WTC vs Pyroclastic Flow

The dust cloud chased people down the street as it expanded, as seen in the picture below.

"A veteran of disasters from the Mississippi floods Mt. St. Helens, Burger said it reminded him most of the volcano, if he forgot he was in downtown Manhattan." "Feeling the heat, seeing the molten steel, the layers upon layers of ash, like lava, it reminded me of Mt. St. Helen's and the thousands who fled that disaster," he said.

"It could have been a tornado or an avalanche or a volcano."

Witness Description Of WTC Dust Cloud [9/11 Mysteries Excerpt]

"When debris started coming down I was right in the shadow of the South Tower.
I was less than a hundred yards away, everybody saw the video over and over again of that cloud chasing people down the street. It was like a tornado, it was like being hit by a wave at the beach, but the wave was intense, it was hot, it was noisy. It was like getting hit in the back by gravel, rocks, like someone had picked up a handfuls of rocks and just thrown them at you. And the noise kept coming and coming and one second I was running, the next second I was flying. I was just umm, I had no control over my feet, no choice as to what direction I was going I was in the air and it seemed like I was being followed by this tornado, this tornado of darkness."

Burnt Vehicles Were Observed At The WTC Complex After The Collapse Of The WTC Towers

Note: While pictures of burnt out vehicles surrounding the WTC are included in this section on the Pyroclastic-like Flow of Debris, it may not be related. It is a mystery what caused the vehicles to catch on fire. The Pyroclastic-like Flow of Debris wasn't hot enough to burn out the vehicles, at least to the best of the knowledge and evidence we have.
There has been speculation that the vehicles essentially exploded due to the EMP Blast released from the nuclear detonations, this seems unlikely to me.
For now the cause of this remains a mystery.