Why does the old turtle dump travelers from his back halfway across the river in The Journey to The West?

When Sanzang and his disciples are kindly carried across the River of Heaven to the West bank by the the Ancient Soft-Shelled Turtle, the Turtle asks that Sanzang ask Tathagata Buddha about his fate. Immersed in his own quest and goals to acquire the scriptures, Sanzang completely forgets to speak to Tathagata Buddha about the fate of the ancient Turtle. Years later, when the Turtle is taking the travelers back across the River of Heaven to China, he asks that Sanzang reveal what Tathagata Buddha has said about when the turtle is to be converted to a human and how long he is to live. Sanzang is deeply ashamed by his accidental betrayal and says nothing in response. Filled with grief and rage, the Ancient Soft-Shelled Turtle submerges himself in the river and leaves the travelers to fend for themselves.

The old, soft-shelled turtle carries the travelers on his back as he walks slowly through the waves for the better part of a day. As he approaches the east bank of the river, he suddenly reminds the venerable Tang Sanzang that he once promised that, if he should ever reach the west and meet with the Tathagata Buddha, he would ask him when the old turtle would be converted and how long he would live.
Unfortunately for the turtle, Sanzang was so preoccupied with fetching the sacred scriptures and with meeting the Buddhas, Bodhisattvas, monks, and other holy men, that he forgot to ask these vital questions. There's a long, embarrassed silence as the old turtle realizes that Sanzang did not fulfill his promise. So he shakes his head and submerges himself into the river with a loud splash, dropping the travelers, their horses, and the sacred scriptures into the water.