My, completely amateur, idea would be that, there are two major regions in China for centralised power. Beijing and Shanghai. Now with the massive growth in the Peal River Delta region, there is a base for a third major region. Currently it is not, because there is no one leader, you have Macau, Hong Kong, Shenzhen, and a large number of other cities in the region.

If Shenzhen, and the people in charge there, are able to become the defacto leaders of the entire region, then you have another centralised regional base, away from Beijing. Which means less power in the central government.

One possible reason: Shenzhen is an economic behemoth that's also a new city (it was basically a fishing village until the 80s), which mean it has fewer 'old hands' there and is relatively free of corruption and socially more liberal (by Mainland standards). In other words, the Communist Party is less influential there than in other major urban centers. Under the reign of Uncle Xi this might be frowned upon.