Switching to DIGI.1 answer
I figured out that the soundbar switches to DIGI.1 because I have a DVR (TiVo) connected to DIGI.1 and it is constantly sending audio to the soundbar. The soundbar detects audio on the optical cable from the TiVo and swtiches back to DIGI.1 unless audio has already started on my HDMI.1 input.
I noticed that after I power up the soundbar, if I first select HDMI.1 and then turn on my device it almost always results in the soundbar switching back to DIGI.1. If however, I first switch the TiVo to the menu (pressing the TiVo button), the audio from the TiVo is silenced and the soundbar does not switch back.
This "feature" of auto-switching to DIGI.1 when audio is present should be turned off.