Depends on how close the sub was to that navy base. Like if the US sub was actually violating undisputed Chinese waters near a naval base, China would be totally justified in firing upon it. Also, it could have been close enough that the PLAN felt it was worth the risk to reveal that they are tracking the sub. assuming that's what happened. It could very well have hit an underwater rocky outcrop (whatever that's called) or collided with a Chinese submarine.

Also, I seem to remember during the Cuban missile crisis, that a US destroyer fired depth charges at what turned out to be a Soviet sub in order to get it to surface to be identified. Of course, the Cuban missile crisis was already an international incident and that sub almost started world war 3 had the commodore of the task force not been aboard to overrule the captain and political officer to fire the nuclear torpedo.