The explanation is surprisingly simple.
During periods of heavy monsoon rain, strong winds can race through and around the mountain pass. When the upward-moving air meets water falling over an exposed cliff, it can break the stream into droplets and spray and push them back towards the rock face.
From a distance, this creates the remarkable impression that the waterfall is travelling upwards.
The water itself has not stopped obeying gravity. Gravity continues to pull it down, but the wind can exert enough force on the small droplets to carry them upwards or sideways before they reach the ground. In particularly strong conditions, the falling water becomes a fine mist, making the effect even more dramatic.