After Joseph is sold into slavery, Judah “descended from his brothers and associated with an Adulamite man” (Genesis 37:36). As Rabbi Ephraim Buchwald put it, he was fed up with this Jewish dysfunctional family. But why should Judah, of all the brothers, be the one to leave? After all, Judah was the most effective leader in the story: the sale of Joseph was his suggestion. But perhaps his effectiveness as a leader gave him an extra motivation to get away from his brothers; with Joseph gone, Judah had good reason to fear for his own life.
Abraham had two sons: one of them was chosen to carry on his mission, and the other was sent away. Isaac had two sons: again, one was chosen, and the other was sent away. Against this background, the rivalry between Jacob’s sons makes more sense. They have no way of knowing that all of them are going to share in the blessing of Abraham, so when they hear Joseph snitching on his half-brothers (37:2), see Jacob’s favoratism toward Joseph (37:3), and hear Joseph announce his dreams of kingship (37:4–11), they have good reason to be concerned.
(Note to self: The verse that mentions the snitching immediately precedes the verse that mentions the favoratism. Must see if any commentators pick up on this.)
But with Joseph out of the way, there is a more uncomfortable situation: eleven brothers still compete for their father’s favor, and they can no longer be united by their hatred of Joseph. If the other brothers could conspire to kill Joseph, then they could conspire to kill Benjamin, the other son of Jacob’s favorite wife. From there, as my lovely and brilliant wife pointed out, Judah would be next in the cross-hairs. Reuben, the first-born, had offended his father after Rachael’s death (35:22), and his weak response to the plot against Joseph (37:21–22) doesn’t demonstrate much leadership skill. Simon and Levi, the next two sons of Leah, had offended their father by how they sacked Shechem (34:30–31). Judah, as the next one in line, might have decided that he didn’t want Simon and Levi to know where he lived.