Why do some nouns change their vowel in the plural (umlaut)?
Umlaut (ä, ö, ü) in plurals is a historical linguistic development. It often occurs with plural endings like -e or -er, especially in masculine and neuter nouns. There's no simple rule for when it happens, so it's part of memorizing the plural form.
Example
der Apfel → die Äpfel, die Mutter → die Mütter, das Haus → die Häuser