So with the trailer for Book 2 out, and many gif sets posted. I have not seen one trying to figure out who is throwing Tenzin, Kya, and Bumi off this cliff.
The red outfit leads me to believe that it is someone from the Fire Nation. The facial expressions show that this came from nowhere meaning they most likely knew this person and were unafraid of them (for the most part at least). Finally, after Tenzin leaves the frame you can see some white (hair?) near our mysterious Fire Nation person.
All this leads me to theorize that the person throwing Aang’s children off a cliff is no other than Fire Lord Zuko. It is possible because to my knowledge, Zuko is still alive during the events of Korra, and because they are Aang and Katara’s children, they knew Zuko and were on good terms with him and his family.