Was Buddhism a rebellion against the Aryans?
When Gautama left his family to become an ascetic or śramaṇa, he joined many other ascetics or wanderers who were separate from the Vedic-Brahmanism religion. 

However, when Gautama left his family to become an ascetic, his goal was not political. The Pali scriptures describe when Gautama lived the life of a prince in his palaces, he was very unhappy &amp; disenchanted &amp; also saw how his rich relatives &amp; associates suffered about worldly things, such as money, status, sex &amp; sensuality, which brought little happiness &amp; much trouble &amp; suffering.   

Therefore, Gautama became an ascetic for the sole purpose of finding a solution to suffering for himself &amp; for the world. Gautama's sole purpose was a Noble Search for Nibbana.

Later, after enlightenment, when the Buddha became famous, the scriptures report many Brahmans ('Aryans') came to visit the Buddha to ask him questions about 'caste' or 'birth' ('jati').   

Because the Buddha was an honest &amp; ethical man, he spoke the truth to the Brahmans (for example, here, here and here. 

In Singala: https://suttacentral.net/si/mn93 ~ https://suttacentral.net/si/mn95
~ https://suttacentral.net/si/mn98

Because the Buddha &amp; many of his enlightened disciples were not from the Brahman caste, the Buddha understood with right wisdom that the highest, most peaceful, wise, virtuous &amp; free state of mind was not related to birth caste (read Brahmanavagga). 

I would speculate that hundreds of years later, under King Ashoka, many new scriptures were written that were very prejudiced against Brahmans (such as here &amp; here). 

I say this because in many scriptures, the Buddha is very civil to Brahmans and encouraged them to follow their own religion in a better way. The Buddha did not try to convert anyone to Buddhism unless he knew Buddhism was the most suitable path for that person.