While Israel lived in that land, Reuben went and lay with Bilhah his father's concubine. And Israel heard of it. Now the sons of Jacob were twelve.
Genesis 35:22

It was common practice in the ancient world for a usurper to take the deposed ruler's wives in a show of dominance. Reuben sleeping with Jacob's concubine, Bilhah, was a failed coup attempt. By taking one of his father's wives, he was making a claim to his father's place in the household. He's fortunate that he only lost his place as firstborn and not his life.