XR18 automixes by itself?

Hello. I have a rather interesting problem/discovery.

There is 1 microphone and 6 line in jacks coming to my XR18. The MIC sounds always the same. But other inputs constantly change level. When I talk loud, the music gets quieter by itself. And when there is no talk, it suddenly becomes much louder. Automixing feature is not enabled. Compressor and other effects are not applied.

The idea was to create several snapshots for each scenario. But I still have to always change the volume on line in channels, as they go up and down.

Is this how mixing should work? Am I mishearing it? Or am I loosing it?