Tanks are expected to keep running in high stress environments. This is either really bad luck or just plain bad quality control, or even worse just plain bad quality period. If the 096B is to be the workhorse tank of the PLA, it had better be just bad luck....
This was an obstacle course especially designed to be testing to tanks.
The whole point of the hazards were to force tank crews to really think about how they approach them. Come at one course too slow and you will stall out; come at another too fast and you can flood the engine or break a wheel etc.
Winning such courses is about cutting your safety margins down to fractions, but when that happens, the slightest miscalaculations and/or mischance could cause you grief.
You also have to remember that the crews have been pushing their tanks hard on that course for extended periods, so that will add a lot of cumulative stress onto the tanks very quickly. They could have been pushing the tank beyond normal tolerances every single time through the course, and done a little bit of damage each time until the final straw that broke the camel's back.
If this had happened first day of the event, the balance of probability would have been on poor quality control and/or crew training, but to happen during the last, grand final race, over a hazard pretty much designed to break tank drive wheels, well that's far more likely the result of a combination of cumulative excessive stress from the entire competition, added with a bit of competative rush causing the crews to try their luck and push their tank just a little bit harder to eek out more speed.
TBH, I'm actually pleased the crews have the daring to do that, rather then being excessively reined in by a fear of having an 'incident' and preferring to play it safe all the time.
The fact that they tank was able to function almost as well with a broken wheel is also a sign of good robust design.