Here's why:
* It's a brand name: Trix is a brand of breakfast cereal produced by General Mills. Brand names are always proper nouns.
* It's capitalized: We write "Trix" with a capital "T" because it's a specific, unique name.
* It's not a generic term: You wouldn't say "I ate some trix for breakfast." You would say "I ate some Trix for breakfast."
Let me know if you have any other grammar questions!