What you are doing appears to be unsupported (invalid). From
http://www.astro.washington.edu/users/rowen/TkinterSummary.html
"all Tkinter access must be from the main thread (or more precisely,
from the thread that calls the mainloop). Violating this is likely to
cause nasty and mysterious symptoms such as freezes and core dumps."

There have been at multiple posts on python-list and stack overflow from other users that have run into this limitation. I cannot find it in our tkinter docs, but I think perhaps it should be (even though not-thread-safe is the default for any python structure).