The other forms of you should be accepted here if translating from the English as there's nothing in the English sentence to tell you that it's the you plural form:
Por qué tú estás enojado
Por qué usted está enojado
Por qué vosotros estaréis enojados
Should all be accepted.
Me too. I was sure it was because it used the question word but could only find this in researching your answer:
When a question begins with an interrogative word, also known as a question word, a verb typically comes next, followed by the noun. This pattern is common in English as well. https://www.thoughtco.com/placing-the-verb-before-the-subject-3079947
The pronoun "you" in English can be either singular (you as an individual) or plural (you as in a group of people).
"¿Por qué usted está enojado?" = "Why are you (a single individual) angry?".
"¿Por qué ustedes están enojados?" = "Why are you (e.g., everyone else in the room) angry?" If it helps, you can think of the 2nd person plural (ustedes) as the good old fashion Southern (U.S.) "y'all" (you all).
Why you're angry? should be accepted. Whether or not someone in the US says, Why"are you" angry? or Why "you're" angry? both would be accepted in everyday conversations such as, Why are you sick...Why you're sick...Why are you at home instead of working...Why you're at home instead of working...Granted when using "you're" it swaps "are you" around to mean "you are."