Answer
Kamenev fell out with Stalin because he was ousted by Stalin from Soviet leadership. Kamenev was a member of the Bolshevik Party and had been a close ally of Lenin. However, after Lenin's death, Stalin emerged as the dominant leader of the party and Kamenev was gradually marginalized. In 1926, Stalin launched a purge of his opponents within the party, and Kamenev was one of those who was targeted.