This might help:
http://support.google.com/webmasters/bin/answer.py?hl=en&answer=35624
It appears that it is a Google behaviour. The title tag may not be descriptive enough - maybe there are lots of other pages on the site with similar titles...?
EDIT:
Specifically (quoting from the above page):
If we’ve detected that a particular result has one of the above issues
with its title, we may try to generate an improved title from anchors,
on-page text, or other sources. However, sometimes even pages with
well-formulated, concise, descriptive titles will end up with
different titles in our search results to better indicate their
relevance to the query. There’s a simple reason for this: the title
tag as specified by a webmaster is limited to being static, fixed
regardless of the query. Once we know the user’s query, we can often
find alternative text from a page that better explains why that result
is relevant. Using this alternative text as a title helps the user,
and it also can help your site. Users are scanning for their query
terms or other signs of relevance in the results, and a title that is
tailored for the query can increase the chances that they will click
through.
If you’re seeing your pages appear in the search results with modified
titles, check whether your titles have one of the problems described
above. If not, consider whether the alternate title is a better fit
for the query. If you still think the original title would be better,
let us know in our Webmaster Help Forum.
<title>
though.