I’ve actually had this happen to me quite often since upgrading to Cubase 6, so I’m surprised no one else has noticed this! I just caught it happen so I thought I should report it.
Occasionally tracks mess up the display of panning and volume information. It’s like the info sent to the display data has been reset to zero or something. The tracks are in fact behaving normally. See the image below:
All audio tracks are the same - all panned center, all volume of -0db. The last two however are showing volume of -infinity and panned hard left - although they still say ‘C’ for center underneath!
To re-affirm, they are NOT set to a volume of -infinity and panned hard left, they still play back as if they were -0 and C panned. They just, for some reason, appear in the mixer as they do. I call that a bug!