Why does my iPhone shift the screen downwards?
This is a feature called reachability. It's built into iPhones so you can reach stuff with one hand.
You can disable it by going to Settings -&gt; Accessibility -&gt; Touch and toggle  Reachability.
Also, if you want to know how to use it, tap the home button twice (some call this gesture a wiggle) without clicking it or swipe down on the home bar.