After running with the wolf, what causes Buck to stop and return "slowly on the back track"?

Buck remembered John Thorton, so he turned to return to the man he loved.

They stopped by a running stream to drink, and, stopping, Buck remembered John Thornton. He sat down. The wolf started on toward the place from where the call surely came, then returned to him, sniffing noses and making actions as though to encourage him. But Buck turned about and started slowly on the back track. For the better part of an hour the wild brother ran by his side, whining softly. Then he sat down, pointed his nose upward, and howled. It was a mournful howl, and as Buck held steadily on his way he heard it grow faint and fainter until it was lost in the distance.

John Thornton was eating dinner when Buck dashed into camp and sprang upon him in a frenzy of affection, overturning him, scrambling upon him, licking his face, biting his hand—"playing the general tomfool," as John Thornton characterized it, the while he shook Buck back and forth and cursed him lovingly.