In Spanish there are two different sounds with the character 'r', the one we call "strong r" and the other the "soft r".
The "soft r" is used when a lonely 'r' is located somewhere inside a word, but not at the end of a syllable, like in "pared" or "granizo".
The "strong r" is used when the lonely 'r' is located at the beginning of a word or at the end of a syllable, like in "rata", or "carta"; but it's also used when inside a word there are two 'r' together, like in "perro" or "carretilla".
"Multirracial" is a compound word, made of the prefix "Multi" and the adjective "racial". As you can guess now, "racial" has a "strong r" sound, so you have to add a second 'r' to keep that "strong r" sound on the compound word. Another example could be "Pararrayos" (lightning rod), made out of the verb form "para" (of the verb "parar") and the noun "rayo".