It's also possible that Google is deciding to not use your titles based on certain keyword searches.
Google will sometimes create its own title if it finds yours to be lacking. If your title tags are short, over-used, poorly written, stuffed with keywords,... they have more chance of being replaced.
It could also just be a timing thing and Google hasn't updated them yet. I would manually fetch a couple pages and check batch in a few days.
Thanks,
Jake