The Official Gmail blog explains the change:
Now when you type something into the Gmail search box, the autocomplete predictions will be tailored to the content in your email, so you can save time and get the information you want faster than ever before. For example, you might now get lax reservation or lax united as predictions after typing “lax” if you have received an email with a flight confirmation for your trip to Los Angeles in your inbox recently.
User-specific autocomplete is rolling out to users over the next few days. If your search suggestions are starting to seem a little more personal, chances are you’ve got it.
Improved search in Gmail [Official Gmail Blog]