What was the foam laid on top of? Propane? If so it's like a fart in a bubble bath. The gas pressure is going to push it through the foam and allow re-ignition. If it is a liquid fuel like gasoline, and it was stepped in, the vapors are given free access to the open air and can re-ignite.
This was a training fire at an environmentally-friendly prop. This prop simulates flammable/combustible liquids fires by bubbling propane liquid through water pans.
Once the propane reaches the water's surface, it is ignited by pilot flames.
With repeated burns, we found that the foam blanket from previous burns sometimes traps enough raw propane in the finished foam to get the phenomena shown above.
The phenomena is more pronounced in cold weather, and it was around 32 degrees F when the photo was taken.
Our prop design allows us to wash the top of the foam blanket out of the prop pans with water fog streams between burns. The propane-bearing foam is pushed to an open area where it is contained and the propane is allowed to burn off harmlessly.
Batwagon and I work together and were running the drill shown, thus my comment to him.
We have tight safety controls, and we don't allow firefighters to enter the foam blanket.
The photo was intended to be a mystery. Apparently, Batwagon doesn't do mystery.