Google changes title tags when a page is relevant for a query but the title doesn't appear to be relevant enough or, in some cases, is too long. There is nothing you can do about it except to possibly re-write your title to be more concise and user-friendly, and then hope that Google decides not to change it in other search queries.
But i've found that Google changing titles can be more beneficial than harmful. i.e. You rank better or look better in SERPs for related terms that you had not originally planned on. You can't stop all instances of Google changing your title, nor can you be certain of knowing all the search terms in which they have decided to alter the appearance of your title.