Often some rap song becomes wildly popular, and it makes use of a novel word combination, or even invents a new word, which then quickly finds its way into AAVE.

Alternatively, many regional words or phrases in AAVE get used by a rapper who gives the word or phrase a wider audience, which then propagates throughout the larger community.

This is a bit of an oversimplification (one could do a linguistics PhD on this topic alone) , but, to conclude, I believe it goes both ways.