I don’t know what causes those flying lights seen each year over the Mekong River in Southeast Asia; but I have a good idea what they are not: They are probably not related to the ropen lights of Papua New Guinea, notwithstanding those mystery lights are sometimes seen flying over water. I believe that the Mekong River Lights are probably caused by a species of bioluminescent insect.
How do they differ from the ropen light? The ropen is believed by some cryptozoologists (like me) to be the bioluminescent glow of giant Rhamphorhynchoid pterosaurs that live in Papua New Guinea and in surrounding countries like Australia, Indonesia, Malaysia, and the Philippines. In 2004, I interviewed many native eyewitnesses on Umboi Island, PNG, some of whom had close-enough views that they could describe to me characteristics; those features strongly suggested a fast-flying long-tailed pterosaur.
The Mekong River Lights, on the other hand, fly slower, emerge from under the surface of the river, and appear smaller than ropen lights: not likely miniature pterosaurs hatching under water. Nevertheless, how wonderful that mysteries still abound in our old world!