Why did the kushites become allies of the hyksos during thier invasion?

To clear things up a bit, when the Kushites became allies with the Hyksos, it was during the Hyksos' reign over Egypt, and the Kushites were independent, NOT during the invasion.

Kushites were afraid that the Egyptians would enslave them again, but they did not know the Egyptians would rise to power again, and the Kushites were encouraged to learn the Egyptian way. Hope this helps.