It's a setting called "Frequent Locations"; as you go about your daily routine, your iPhone makes note of where you are and how long you're there. When it starts detecting patterns, it marks the spot as one such "frequent location." It (rather accurately) assumes workplace location based on where you are during the day and your house address based on where you are at night, and it tracks various repeated locations regardless of time: friends' houses, favorite restaurants and the like.
The word from Apple is that "this data is kept solely on your device and won't be sent to Apple without your consent. It will be used to provide you with personalized services, such as predictive traffic routing."
Here's how to see what your iPhone has identified as your frequent locations, as well as how to disable it.
Tap on "Privacy."
Tap on "Location Services."
Scroll to the very bottom and tap on "System Services."
Finally, here we are. Tap on "Frequent Locations."
Here it presents us with a list of what it considers our "frequent locations" — wherever it records you as spending a lot of time. Let's check out the New York locations.
Without my telling it to, my iPhone has identified my home, my office, several friends' houses, and other places I frequent. If we tap on a specific address...
...it tells us when we were there and for how long.
To disable this data-collection-happy feature, flip this switch in your Settings.