Boilover

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A Boilover type of fire refers to an extremely hazardous situation where an attempt is made to extinguish semi-enclosed oil or petrochemical fueled fire with water. The hazard results due to the difference in density between oil and water. Boilover is also common in the home as a chip pan fire when cooking.

As water is being pumped onto the surface of the fuel (where the flames are evident) it will quickly sink below the fuel due to the water's higher density. The water collects at the bottom of the container and has little effect on extinguishing the flames of the lighter petrochemical fuel. As the temperature of the fire builds, the collected layer of water is heated. The water will eventually vaporise into steam, as its boiling point of 100 °C (212 °F) at standard pressure, is reached. The water will then expand to more than 1700 times in volume, as compared to its liquid state. The rapidly expanding steam (possibly superheated) expels the oil or fuel above upward and out of the container, resulting in the discharging of burning oil onto a large and uncontrolled area outside of the container.^[1]

Oil gets so hot that it catches fire all by itself
Water is poured into the container
Water is denser than oil, so it sinks to the bottom. As the water touches the bottom, it is heated above its boiling point and instantly vaporizes.
The water vapour expands rapidly, ejecting a fireball of burning oil out of the container and into the air where its surface area increases exponentially and combustion proceeds much faster