"Where are you running to?"
Oh I think I understand thanks to my Turkish partner!
Bu nerede? Where is it?
Kedi nerede? Where is the cat?
The "where" is a place.
Kedi nereye koşuyor? Where is the cat running to?
"Where" is a direction. Even if the answer is a noun or place, the answer will be "to" that place: eve not ev ("to home", not "home"), indicating a direction.
In which direction is the cat running? Kedi hangi yöne koşuyor?
Can use "which", but it's a different sentence.
Why does it say neredesin when you hover over
Because "Where are you?" can translate to Neredesin? As in "Where are you located?"
But of course that's not the meaning that these three words have in this sentence -- "Where are you located running to?" makes no sense, and for the same reason, neither does Koşuyor neredesin?.
"are" here does not mean "be located"; it's just a helping verb to form the present tense of "are running".
And "where" does not mean "at which location"; it's part of "where ... to" which means "to which location".
So you need nereye (to which location) and not nerede (at which location).
And then not -sin (you are) but koşuyorsun (you are running).