According to the WTO meeting agenda
They are citing a US non-compliance in 2017 trade dispute ruling as backing for the request. Do remember that if the US is to go forward with the $200 billion tariffs on Chinese goods, China won't be able to retaliate with tariffs of the same magnitude.
I guess they are trying to pursue other avenues to hit back and this is one of them. Certainly something that will trigger some nervousness in markets should they actually go through with it once the US announces those tariffs. For now, they're still "playing by the rules" and going through the WTO so let's see how things will develop from here.