"Why do you rush?"
Translation:Waarom haasten jullie je?
It's a reflexive verb, meaning it takes the same subject as object. When translated as "Waarom haast je je?" (with singular "you"), you can think of the first "je" as being the unstressed "jij" (subject), and the second being the unstressed "jezelf" (reflexive object).
In English, we would say "Why do you rush yourself?"; the difference is that "yourself" is optional in English, but not in Dutch.
Another way to think about it is to rephrase the sentence so that it's not a question. Then there is a clear separation between the subject "je" and the object "je":
Je haast je omdat je geen geduld hebt (You rush yourself because you have no patience)
I wrote 'waarom haas je je' which was not accepted. The response was that there was a typo, with the correct translation given as above. This does not help me to understand my error. I left off the 't' from 'haast' as it is in question form and followed by 'je'. Is this an exception to the rule, or is something else wrong?
It's not an exception, but the rule simply doesn't apply here because the "t" at the end of "haast" is not the suffix -t.
The infinitive is "haasten" with a "t", not "hazen", so one would write "ik haast me" and "haast je je?". Adding the -t might be expected to produce "je haastt je", but that violates the rule that you can't have a doubled consonant at the end of a word, so you instead just use "haast" for that form as well.