Based on what’s known publicly (and the photos I’ve seen of the damage), I’d say it would most likely have been hitting an underwater seamount - either uncharted, incorrectly charted, or the sub screwed up their navigation fix (that’s in descending order likelihood).
Outside of this, maybe (but highly unlikely) she hit a submerged shipping container at speed (thousands of containers are lost overboard every year). The South China Sea is ridiculously filled with lost shipping containers.
Wrong area/time for a whale strike at speed.
If she hit a Chinese (or other country’s sub) they would be screaming about it from the mountains.
good points...