This is tremendously aggravating. I find that this blog mysteriously changes back to the default WordPress theme without warning at completely random times. After turning on lots of logging, I found that the GoogleBot and regular visitors seemed to be causing it, but they caused it when they made completely normal requests from the site.
After getting flustered, I realized a dirty hack was in order. I made a new theme folder called 'default-original' and moved the contents of the default theme into it. After that, I copied the contents of my desired theme folder into the default folder. After waiting a few hours, the magic happened and the theme was reset. But alas - the default theme is now my desired theme.
Dirty hacks sometimes work the best. 
