From Internet chatter, it seems that the cryogenic second-stage YF-75D engines prematurely shut down, leading to a late separation of the second stage from the first and the eventual inability to reach the designated orbit.
Interestingly, a similar incident occurred during the maiden launch of the LM-5 in November 2016, in which the (supposed) second stage put the YZ-2 third stage and its payload on an incorrect orbit that was later rectified using the YZ-2. These two anomalies could be related and warrant further investigation.
Better now than during the upcoming Chang'e-5 launch.