most logical interpretation: kathmandu valley was a glacial lake during the Pleistocene Period. At the end of the ice age the glaciers melted and water with its sheer pressure started eroding the surrounding mountains. Then one fine day, after it had been successful in creating a outlet for itself it flowed out from the valley towards the sea. maybe manjushree with the help of other chaps dislodged a huge rock and the water flowed out.