The film and foam layer applied to a fuel or fire will begin to breakdown and lose the original qualities.
Foam solution may have to be continuously applied or reapplied until danger of a flashback is eliminated.
Anything that disturbs or diminishes the film layer, even though it is "self-healing" with AFFF, may cause enough surface area to release vapors and if the ignition factors are favorable...... FIRE MAY REOCCUR!

Weather

Past - Current - Expected

Hot & cold effects

Rain will breakdown foam

Radiant heat
from other fire

Wind can blow foam away

Conditions change during day to night

AND may cause fuel to flow out of containers or to other areas!

Time

As time goes by,
so goes the foam

If it is freezing temperatures,
what is happening to the water portion of your foam blanket?

Fuel type

Hydrocarbon
Vs.
Polar Solvent

Specific effects from each product. Is it the right foam?

Original foam quality & application technique