High Sierra Airplay Problems
That's probably because High Sierra introduced AirPlay2 (before, you were running AirPlay). With AirPlay2, it's no more a system wide output that's being redirected to your remote player, but instead a per-application output. Technically speaking, it's now a userspace library that application must implement.

Since only iTunes supports AirPlay2 for now, I guess this is the reason why it's being launched.

As to how to disable this, please check if your player can disable its AirPlay2 announce or use iTunes...