Question

Why does the old turtle dump the travelers from his back halfway across the heaven reaching river

Answer 1

The old turtle is a character taken from the Chinese novel 'Journey to the West'. The old turtle roams around the east bank of the river, upon seeing Tang Sanzang a Buddhist monk in the bank; old turtle asks him to ask Tathagata Buddha when he would be converted and how long he would live. But upon seeing Lord Buddha Tang Sanzang forgets to ask and this makes old turtle angry and he dumps the fellow monk in the river for not granting his request.

Answer 2

If you are talking about the journey to the west, it is because the turtle is angry that his question was not asked to Buddha.