They were both followers of the guy who started the nation of Islam (different from actual Islam) who got jealous of Malcolm X being a bigger star of the group (and worried Malcolm would spill the tea about the dude misappropriating funds and sleeping with a bunch of female adherents), so the guy excommunicated Malcolm and told everyone Malcolm was basically evil… Leading to Mohammad Ali turning against his friend… And led to the eventual assassination of Malcolm X.

I highly recommend Malcom X’s autobiography to anyone who hasn’t read it, incredibly fascinating.