iOS lets apps declare specific background modes such as audio, location, or VoIP — using one of those modes for a purpose it wasn't designed for, just to keep the app alive in the background, is a violation.
Common when an app declares the "audio" background mode to keep running for an unrelated reason like syncing data or tracking analytics.
"Your app uses background modes for purposes other than their intended use, per guideline 2.5.4. The 'audio' background mode should only be used for continuous audio playback."