no, it all depends on the emphasis
- "Bana bunu neden söylüyorsun?" (you ask the reason of "asking", emphasis is on the verb)
- "Neden bana bunu söylüyorsun?" (you ask the reason of asking THIS)
- "Neden bunu bana söylüyorsun?" (you ask the reason of asking this thing particularly to YOU)
- "Neden bunu bana sen söylüyorsun?" (you ask the reason why it is particularly THIS PERSON asking you the question)
etc etc etc
I guess from the explanation above the emphasis is on the word before the verb. So in the sentence you gave, because the question is actually :" Why are you asking about this particular thing", -bunu- was placed before the verb for emphasis. However, in the first case : "Bana bunu neden soyluyorsun?" , because the question is about the verb itself and not anything else in the question, neden was used to emphasize that. I guess that this is a special case of emphasis. My explanation could be wrong, I am not a native speaker and I am myself still learning, so please correct me if I am wrong. This is how I understood it though, hope this helps a little!
a) it's probably not incorrect to reject that sentence as a translation for this one
b) if there's an error, the project team will see it a lot faster when you Report it rather than posting a comment
c) the project team is all volunteers who work on this in their free time - since we're not paying for Duolingo, there's no money to pay salary to people whose full-time job is monitoring submissions and reacting within a certain time.
jytou, "why do you tell me this?" is wrong because the verb of the sentence in Turkish is not at the simple present tense but at the continuous present tense. At the present simple tense it would be "bana bunu neden söylersün?", verb söylemek at the thrird person singular. Look at there: https://www.verbix.com/webverbix/go.php?&D1=31&T1=s%C3%B6ylemek. You find every verb's conjugation at every tense. Very helpfull.