Matt Cutts did a video about what criteria they use to change page titles back in March, and Jennifer Slegg posted a breakdown at http://searchenginewatch.com/sew/news/2342232/why-google-changes-your-titles-in-search-results.
Cutts mentioned 3 criteria that they use. If your existing title doesn't meet that criteria, they may change it.
1. Concise (short enough title)
2. Good description of page and site that the page is on
3. Relevant to your query
Apparently, Google finds it more relevant to put the Category name + the name of your site.
