Personally, I think it's firmware. If you read through the thread this only happens with this lens, but also specific camera settings.

It only happens in a 20D and AFTER the 2.0.2 firmware upgrade from May, 2005
I shot with this lens prior to the upgrade and this did not happen.

I just posted this ....
Turning ON the ISO expansion ends the "phenomenon."

Quote - How bizarre! This must be an electronic  ... I would take the lens back to Tamron and get it rechipped has to be the easiest option if you want to save space on your card... Or put the lens on ebay maybe somebody would like that feature!! Be interesting to know if the lens does the same on another 20D