It could be that Apple found a crash that your previous testing did not. They are more focussed on finding crashes than normal testers, who are concerned with other things like functionality and ease of use. Apple are not as bothered if your app is poor quality or if it does not always work as advertised - that is taken care of by app store reviews and ratings.
So consider doing more testing focussed on getting it to crash. Run it without an internet connection, or with a very poor one. If it an audio app (from its name) then how does it interact with background audio or if put in the background? Feed it bad data and see how it handles it. What about if a user has a bluetooth keyboard connected, or some other peripheral? Or is running it on a device with Chinese as the default language?