A thread from Sept, 2010... and it's been going on much longer than that....
Google is testing altered page titles
Google has been testing different title rewrite approaches over time, I think incrementally verifying each approach before going ahead to the next one.
I also think they're also using rewritten titles to make pages more prominent as they test different algo factors and winnow out pages that perhaps don't fit certain types of user intent. So I may be disagreeing with some of what I'd posted in the above-noted thread... It's possible, I'm thinking, that the title rewrite and the ranking change might be happening in tandem.
It's not always good news if you've got a page ranking for a query that really doesn't fit it.