Google's "Modified Titles" is an aspect of their ranking results where they push content via their snippets that is tailored to the search query. This means that although you don't have a Meta Description or you have a Title tag already filled in, that they may include a snippet description to fill in for your missing Meta Desc tag and they can even re-write your Title based on how your page's content relates to the specific user query!
Take this quote from their Webmaster Resources page: Site title and description
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.[emphasis added]
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.
Well, how can you influence what Google's going to modify your Title to if you don't know what the specific query will be? The answer is easy... know what the specific query will be!
Well, in the real world we can't guess every query that people are going to put into Google but we can get an idea of them and if they're even close to the actual query that's being used, Google will likely go off of that.
You see this whole movement of Schema Microdata Markup is allowing for more concise targeting of content based on topic and allowing for the definition (to the engines) of your site's relevance to that specific topic. You're able to tell them what you're relevant to.
So here are some tricks to finding out what queries are being searched for in Google that relate to the content of your webpage:
Google Analytics - Navigate in GA to Traffic Sources > Sources> Organic (and Paid if you're doing a paid campaign) and you'll see the keywords, keyterms and keyphrases that people used in Google to find you.
Let's start with the keyphrases (i.e. actual long search strings that appear as sentences or questions like "safest car seat for 3 month old baby" or "who is the best golfer in the world?"). Separate these out into a spreadsheet under a column entitled "keyphrases".
Next pull in the keywords and keyterms (keywords are just words put in to help define to Google what you're looking for like "car, audi, black" while keyterms are strings that logically go together as in everyday usage, though they may not be complete sentences, such as "long-haired dog" or "basketball players").
If you find that you're getting a lot of traffic for "best golfer" then take this and add it to your spreadsheet under a column entitled "keywords/terms".
Then the next step is to create questions or phrases out of these top terms based on the keywords that people are using. If you see that people are searching for "best golfer", "top golfer PGA", and "world's greatest golfer", etc then you can create questions based on these and incorporate them into your content.
Create questions like: "Who is the world's greatest golfer: Well that honor goes to..."; "which golfer won the PGA tour last year?: Joe Shmoe won by such and such...". By actually including search phrases and questions in your content as opposed to just keywords and terms then you will be allowing Google to pull from those and present your answers as potential answers for their users who are searching for them. Separating the question from the answer with a ":" might help Google grab just the question for the Title and the answer possibly for the snippet (which is often pulled from the Meta Description tag). Google recommends that you use Microdata to markup the content to help them define that part of the content as likely the best for the snippet. (Click here if you want to know more about Microdata)
Another great tool is Google Webmaster Tools which gives you actual queries used in Google where your site came up (whether clicked or not) and what your average position is.
I also use Google itself by doing a keyphrase search (either statements or questions) and either check out the Google suggest dropdown or look at the bottom of the 1st page results for suggested related searches.
Another awesome tool is the Contextual Targeting Tool in Adwords. Put in a keyphrase and see what it recommends as related.
Go to some of the bigger answer websites like About.com or Answers.com and do some searches for questions. They'll show you a list of related questions that they're being asked.
If you want you can also create a topic-specific FAQ at the bottom of a relevant page. If your page is about the PGA Tour and you want traffic based on that then create a little FAQ or trivia section and include some of the more popular search questions.
Remember to mark-up your content with Schema.org Microdata.