Maybe China figured that detonating nukes in that area unannounced will prevent other countries from meddling there. Kinda makes sense.

Nov 18:
"China calls on US to 'stop flexing muscles' in South China Sea"

[link to www.straitstimes.com (secure)]

Nov 18:
Thread: Chinese aircraft carrier on the way to South China Sea

Nov 20:
"US to provide Vietnam with coastguard ship to patrol S. China Sea"
[link to www.rt.com (secure)]