One potential factor is that if your title is very short Google is more likely to rewrite it, since it may class very brief titles as non-informative - or at least, less informative than they would be with more words. It will then look to external sources as well as on Page cues to amend the title.
What I'm driving at is that Google is discounting your own content in favour of someone else's. This heavily implies that Google believes the third party is a more accurate description than the one supplied by your own site. This implies either a problem with your content that makes Google consider it not relevant, or a lack of strength in your site that makes an external vote sufficiently strong for Google to use it.
The only other possibility would be something like a 302 hijack type of thing: