Knew the captain for a long time. He was a good guy.
About the freezing rain that could be a possibility and the twin otter is very old aircraft with no anti ice capability. And it is common for the pilots to fly into IMC without proper IFR routes in the terrain based on heading and GPS. NAC airlines is the only airline that flies the Kathmandu to Jumla via Pokhara Route. I have flown in that terrain but never encountered icing.
But they are many possibilities. Without a CVR and FDR on board, no one would know. And usually they blame it on the pilot as always. Sad but the truth about aviation in Nepal.
Safety needs a huge improvement!!