This midrash fills in a major blank in the biblical text: when Samuel comes to anoint a new king from Jesse's sons (1 Samuel 16), Jesse presents seven of his sons, but leaves David out in the field tending sheep, not even considering him a candidate.

Here is the story behind why David’s family treated him like an outcast.

The Lineage Crisis: Ruth the MoabiteThe scandal starts with David’s great-grandmother, Ruth the Moabite. Deuteronomy 23:3 states that no Moabite may enter the congregation of Israel. While the Sages later clarified that this law only applied to Moabite men—not Moabite women who converted—the issue was heavily debated during the era of David’s father, Jesse (Yishai). Jesse was a religious leader and descendant of Boaz and Ruth.

Midrashic sources explain that midway through his life, Jesse grew anxious about his own lineage. He worried: “If the law against Moabites applies to women, then my great-grandmother Ruth’s marriage to Boaz was invalid. That means I am technically not a kosher Jew, and my marriage to my Jewish wife is forbidden.” The Separation and the PlanTo avoid potential sin, Jesse separated from his wife—whom the Talmud identifies as Nitzevet bat Adael—after they had already had seven sons.

Years passed, and Jesse desired another child. He decided to take a non-Jewish maidservant (Canaanite slave) on the condition that if he was not a valid Jew, she would remain a maidservant, but if he was a valid Jew, he would free her first. The maidservant, feeling immense pity for Jesse’s heartbroken wife Nitzevet, concocted a plan reminiscent of Rachel and Leah: On the night Jesse was to meet the maidservant, Nitzevet secretly switched places with her in the dark. Jesse was entirely unaware of the swap, and Nitzevet conceived.

The Apparent "Mamzer"Three months later, Nitzevet’s pregnancy became noticeable. Because Jesse knew he had not been intimate with her in years, his sons assumed she had committed adultery. Nitzevet refused to reveal the switch to protect her husband from public embarrassment. She quietly bore the humiliation, and when the baby was born, Jesse accepted the boy into the household—naming him David—but assumed he was another man's son.

As a result:He was treated as a outcast: David was not raised with the privileges of his brothers. He was sent away to the dangerous wilderness as a shepherd—far from the family home. He was presumed a mamzer: He was seen by his brothers as an illegitimate child of adultery, making him a social outcast.

Clues in the Book of Psalms

This midrash is used by Rabbinic commentators to explain several cryptic verses in the Book of Psalms attributed to David: Psalm 69:8: "I have become a stranger to my brothers, an alien to my mother's children."

(The word used for "stranger" here is muzar, which shares a linguistic root with mamzer. Notice he specifically says his "mother's children"—because his brothers shared a mother, but believed they didn't share a father.)

Psalm 51:5: "Behold, I was brought forth in iniquity, and in sin did my mother conceive me." (Often misinterpreted in Christian theology as "original sin," rabbinic sources view this as a reference to the public shame surrounding his conception, as everyone believed his mother had sinned.)

Psalm 118:22: "The stone that the builders rejected has become the chief cornerstone." (Refers directly to David being completely disregarded by his father and brothers, only to be chosen by God to be King).

Vindication

When the prophet Samuel arrived in Bethlehem to anoint Israel's next king, God rejected all seven of Jesse’s older sons. Samuel asked, "Are these all the boys you have?" Jesse reluctantly admitted there was one more—the shepherd boy. When David walked in and God told Samuel to anoint him, the secret of his birth was finally revealed, publicly vindicating Nitzevet and establishing David as a fully legitimate son of Jesse, the rightful King of Israel.

(http://www.autoadmit.com/thread.php?thread_id=5886889&forum_id=2̃#50028570)