These stops are fairly common for trains running for over 10 hours. The main reasons are two-fold: maximum working time for the drivers, conductors and crews, and administrative planning.
After several disasters due to driver fatigues, drivers and train conductors may not work for over 8 hours before having a required rest period (10 hours in certain cases; but 8-hour shift remains recommended and the rule of thumb).
A shift change for drivers will be required in Xuzhou in any case.
Although this alone usually does not require a 28-minute stop.
There are additional jurisdictional and administrative considerations.
Chinese Railways are divided into several Railway Administrations/Bureaus (sub-companies nowadays), each having "jurisdiction" over routes in a given region.
In principle, the trains within the region should be run by staffs from that region. There are many exceptions and this is no longer a hard rule nowadays but some effects persist.
Within the bureaus, there are also sub-divisions over particular routes/tracks.
The Beijing-Shanghai route involves two administrative bureaus, Beijing and Shanghai.
Xuzhou is also the first stop in Jiangsu, which is Shanghai Bureau, after Dezhou (Shangdong, in Beijing Bureau).
Xuzhou is also one of the most important rail hubs in China.
At the intersection of Longhai railway and Beijing-Shanghai railway, it serves as an important connection point.
It also has facilities that can carry out many maintenance operations if necessary.
The sister train T110 (in the other direction, Shanghai-Beijing) has a long stopover in Changzhou instead, a couple hours after departure from Shanghai. Before the stopover was in Nanjing.
If I had to guess, I would say it's to reduce the burden on Xuzhou station, which is already overloaded.