GoKawiilPrioritization is the reason why Android, and by extension Android Auto, will automatically drop the volume. It's a focus system that gives priority to a particular app over another for a limited time only.
Here's an example. You're listening to music, so that's the primary focus while it's playing. However, you then receive an SMS notification. Android makes sure that you don't miss that notification. It briefly reduces the volume for your music so that the sound for your notification is heard clearly.
This is called audio ducking. It's been an Android feature since release 8.0. While it isn't a universally-loved feature, it exists for clarity, giving you a chance to hear when important notifications or alerts appear without too much impact. It's designed to be temporary, and should only impact your audio for a few seconds.
A rarer cause of the issue, however, might be Android's hearing safety features. Again by design, and introduced with Android 14, Android will try to protect your hearing by tracking loud audio and automatically lowering the volume for you if you listen to it for too long. Android will tell you if it's automatically doing this, and it's a feature, not a bug.
You can toggle the media volume limit setting off if this is the case, but it's worth taking note of the risk and using a volume level that isn't likely to cause damage to your hearing.