This wouldn't be the first time that China has held up a major tech transaction. Back in 2012, the Chinese government held back on approval of Google's acquisition of Motorola until the former promised that it would keep a free licensing policy for the Android OS. Now, the Chinese government is worried that once the Microsoft purchase closes, Nokia will use its patents (which it retains in the deal) to bring in some revenue by forcing Chinese handset makers to pay licensing fees.
Nokia has not gone after manufacturers in the country, some of whom have possibly been infringing on its patents, since it was selling smartphones in China. Once smartphone sales belong to Microsoft, there is nothing holding back the Finnish based firm from demanding that Chinese OEMs sign licensing deals in order to use Nokia's patents. The Chinese manufacturers are using this situation to request that the Ministry of Commerce demand lower patent fees from both Nokia and Microsoft, using approval of the pending transaction between them as leverage.