I explore the conjecture, first floated informally by Professor Michael Witzel, that the Malla, the Sākka, and the Vajjī tribes (amongst others) were late arrivals from Iran.
They seem to have settled first near Rajasthan, where they rapidly assimilated and took on an Indic language. But they retained ideas from Iranian, and more specifically from Zoroastrian, culture. Then possibly about 810 BCE they moved East and North into the Ganges Valley, where they settled on the margins of the existing kingdoms (Kosala, Kāsī, and Magadha).
This is rather speculative and based on some stray references in Pāḷi. On their own they are tenuous, but taken together they seem to add up to a plausible argument.
Many of these tribes were socially organised differently from either the Ganges Valley kingdoms or the Brahmanical village-based system. Oligarchy (rule by a group of older men) seems to have been common.
I’ve further speculated that Zoroastrian beliefs interacted with the local worldview (already a syncretism between Indo-European and Chthonic beliefs) to create the basic Buddhist worldview. Also that the incomers influenced Brahmins in Kosala at the same time. They were already breaking away from the orthodox Kurukṣetra Brahmins and thus created the ideological revolution visible in the Bṛhadāranyaka Upaniṣad.
The article is free online if anyone wants to read it.