Why does my iPhone sometimes start rendering from a third of the way down the screen? I don't really know the right words to describe this problem, but every once in a while (sporadically, maybe about twice a day), whatever view is displayed on my iPhone (iPhone 6, iOS 8.3) screen will slide down to about a third of the way down the screen, as shown in these three screenshots:



(I've whited out personal data.)

After a matter of seconds, I think it slides back up on its own, though I've discovered that the most convenient way for me to cancel this phenomenon is by pulling the notification center into view from the top of the screen and then pushing it back up, which causes the main view to slide back up right away.

What is going on here? Has anyone else seen this?

Update:

Today I learned that I fidget with my thumbs "sporadically, maybe about twice a day".
It's the "so I can reach it with my thumbs" option, called Reachability

Double-tap the Home button to enable.
Tap the screen to disable.

The double-tap is not a double-press. You don't engage the switch in the button, just tap it lightly.