I second what "AeolianReflex" said.
This looks like the adhesive is giving way. This can be simple case of bad assembly, or screen exposed to unusual heat. Left in the car or on the table on a hot day.
It only takes simple hairdryer to soften the adhesive and separate digitizer from the glass, so car heat or direct sunlight is more than sufficient to do this to a phone.
If you still have a warranty, I would go ahead and claim it. It's not likely something you can fix yourself.