One month decline amidst competition may be a statistical artifact. Update: 500K adds in Jan 2018
In December 2017, China Unicom fell from 77,503M to 76,539 broadband customers. That's still the third highest in the world, three times as much as Comcast.
Competition is a natural explanation. China Mobile has been adding ~3M/month for the last six months and 35M on the year. China Telecom added 660K in December and 10.4M for the year. China Telecom now has ~150M subs and China Mobile 113M. Nearly all (>90%) at CT & CM are on fiber all the way home.
China has > 800M LTE subscriptions, with many of the 200M 3G customers soon to migrate. LTE speeds are often 50-150 megabits. It's inevitable that some people will go wireless only; 5M have chosen wireless for convenience in Japan even though fiber was available. But China Mobile and China Telecom continue to grow rapidly.