My guess is that you haven't really exited Thunderbird before departing for work, and it's automatically checking/downloading email on a regular schedule throughout the day. When you get home and start it, you're really only switching to the already active program.

Check the "Processes" tab in Windows Task Manager before leaving for work and see if Thunderbird is still running. I know you said that you did that, but I can't tell whether you did it after you got home or in the morning before you left for work.