The most likely reason is a requirement to make an emergency landing soon after take-off. This could be due to an engine fault, a bird strike or a medical emergency, and since maximum take-off weight is far greater than maximum landing weight, the pilot needs to dump fuel.
For example topped up, an Airbus A380 carries 250 tonnes of fuel. While the maximum take-off weight of the aircraft is 575 tonnes, the maximum landing weight is 181 tonnes less. Landing with a heavier aircraft than that figure risks buckling the undercarriage, with potentially catastrophic results.
If an A380 were to take off with close to a full payload and very soon afterwards was required to make an emergency landing it would need to dump more than 150 tonnes of fuel. Fuel dumping would usually take place under instructions from the control tower, preferably over the sea or a rural area at an altitude high enough for the fuel to dissipate rather than allowing it to rain down to the ground in liquid form. Even so, the fuel vapour will end up in the atmosphere, and ultimately return to Earth in the form of rain.
Ideally, the minimum altitude for a fuel dump is around 2000 metres. Fuel is ejected at the rate of several tonnes per minute through valves on the trailing edge of the wings, close to the wingtips and away from the engines to avoid combustion.