When I modified some page titles and shortened them Google responded by using my new title AND they also added my domain name to the end of the title that showed in the serps.
Were your shortened page titles an exact match for the search term and were there any other pages in the SERPs with the same title ?
I have seen this with my site and assumed Google was amending the title results in order to avoid returning a results page with lots of identical titles.
I have subsequently changed my titles to include my domain name at the end of the title. I don't check my rankings that closely so can't really tell if it has made a difference either way. However I figure it helps with branding and even if I am at position 8, for example, and I don't get the click at least my brand name gets an eyeball.