The most popular story is that Kaikeyi's maid poisoned her mind. Due to this, she sent Lord Rama on a Vanvas.
Although, I heard a different story in a Katha recently. The story goes like Lord Rama was to be coronated as the king. Although, he knew this was not his purpose of descent (of being an avatara). He shared this dilemma with Kaikeyi. Kaikeyi suggested the solution that she will send him on Vanvas. Rama said that she will become infamous for thousands of years to come. Although, Kaikeyi said that she was fine with that as long as she was born as his mother in the next birth too. Hence, Lord Rama was born as Lord Krishna, and Kaikeyi took rebirth as Yashoda.
Is there any version of the Ramayana that mentions this story?