Ahh.. got it to stop doing that, by one at a time going into each "source" and turning off "AutoChange Visuals" for each in turn.

The FM Tuner had it enabled, and as soon as I turned it off there, the DSP (player) stopped doing AutoChange. I can now turn it on and off and it works normally.

So what I think was happening is perhaps a bitfield extraction bug in the player software. The AutoChange settings are stored together in a single byte, with 2-bits for each of four sources:

unsigned char vis_autochange_dsp : 2;
unsigned char vis_autochange_aux : 2;
unsigned char vis_autochange_fm : 2;
unsigned char vis_autochange_am : 2;

The autochange_fm was set to 01, and somehow this affected autochange_dsp. So it must be some weird bitfield extraction or sign-extend bug.