Reuben had sexual relations with Bilhah. This dishonored his father's marriage. As the oldest son, Reuben might have been trying to take over as the clan leader using a pagan method (compare 2 Samuel 16:15–22). But this act caused him to lose his birthright (see Genesis 49:3–4).
Jacob soon found out what happened, but he delayed his response again (see 49:3–4; compare 34:5).