As per Chandogya Upanishad 5.3, Śvetaketu, son of Gautama Rishi, went to the court of the Pañcālas where the prince Pravāhaṇa, the son of Jīvala, asked him a series of questions related to adhyatma-vidya. Svetaketu didn't know the answers and narrated the same to his father who too confessed his lack of knowledge in this matter. Gautama then proceeded to the court where he was told
O Gautama, regarding the matter which you asked about, no brāhmin before you had access to this knowledge. This is why in the past, in all the worlds, it was only the kṣatriyas who had the right to impart this knowledge.’ Having said this, he proceeded to teach Gautama.
My question is how come kshatriyas had knowledge of adhyatma-vidya which is arguably the domain of brahmins ? Is there a story behind it ?