Provide a plausible explanation for the flash fire that occurred during the unapproved rinsing procedure described in the case study below.

$(2$ marks$)$

Case study: Ignition after rinsing a drum A drum was used to store an aqueous $($thus conductive and non$-$flammable$)$ solution. The drum was made from non$-$conductive plastic and its volume was $200$ litres. After emptying the aqueous solution from the drum, an operator decided to dry the drum by rinsing it with some acetone $($a volatile and flammable organic solvent$)$ and then letting the flammable solvent evaporate. Even though this procedure was not officially approved, it was quite a common practice on the plant. To perform this operation, the operator poured two litres of acetone into the drum and forced the drum to roll over the ground in order to ensure good mixing of the remaining water with the acetone while thoroughly wetting the internal walls of the drum with the solvent. Once the operator thought the drum had been rolled enough he emptied it by turning it upside down and pouring out the acetone onto the surface of the concrete floor. Afterwards, the operator performed the same procedure with another water$-$wet drum. When the operator emptied the acetone out of this second drum a flash fire occurred in the area where the acetone was being poured $($see Figure $3)$