Perhaps the speaker thought the person was arriving - for example they are at the door of a party in a coat... but they are leaving very early, so arrival rather than the journey (coming applies to the whole journey) is emphasised. However the English is awkward. We would say 'are you not (aren't) you arriving?" Or the speaker is verifying that the journey didn't reach its conclusion - more naturally 'you don't arrive?' but 'you do not' could be used (now that i think about it this is a sort of surrogate past tense in English asking about a past narrative if that makes sense).
I appreciate your answer, it gives me a glimpse of the English language. I am a native Romanian, I would not ask "voi nu sosiți". If they did not conclude their journey I would ask "When are you going to arrive?/ Când sosiți?" I wrote the comment to let them know that they have to change this sentence.
You are right! It does get very complicated when trying to figure out the context of some of these sentences! We could also ask 'Aren't you coming' or 'when are you going to arrive' or 'when are you arriving' all seem more natural options that the English suggested by Duolingo.