Learn Polish: Z powodu uprzedzeń jego zachowanie wobec cudzoziemców jest okropne. - Due to prejudices, his behavior towards foreigners is terrible.

It is required by the preposition wobec, which means 'towards' or 'in relation to'. As covered in this lesson, whenever you talk about an attitude, behavior, or feeling directed at someone using wobec, the noun that follows must be in the genitive case.

How does the base word cudzoziemiec (foreigner) change into cudzoziemców?

Cudzoziemiec has a 'fleeting e' in its final syllable. When adding most grammatical endings, that e disappears, making the stem cudzoziemc-. Because it is a masculine personal noun in the genitive plural, it takes the standard -ów ending.

We also use the genitive plural for uprzedzeń (prejudices). What triggers it here?

The phrase z powodu (due to / because of) always requires the genitive case. It literally translates as 'from the reason of', and the noun describing the cause must follow in the genitive.

What is the base form of uprzedzeń, and why does it look like it has no ending?

The base form is the neuter noun uprzedzenie (prejudice). In the genitive plural, neuter nouns ending in -nie typically have a 'zero ending', meaning they drop their final vowels. The -ni- sound shifts to the soft consonant -ń, resulting in uprzedzeń.