It's the base influencing the superstructure influencing the base etc. a lot of the islamophobia can be traced to the crusades, later west europe's fight with the ottoman empire (people tend to forget that it laid siege to Vienna, twice!), it got used and repurposed through the ages and is currently being stoked by the bourgeoisie to divide the labor-class into a labor aristocracy and a proletariat. If you wanna know why an individual is racist it's obviously gonna differ from person to person but a lot of the tropes can easily be traced.

The "muslim/asiatic horde" for instance even etymologically comes from the ottoman "ordu", the turkish word for army.