Bad Glue + Bad Battery
The glass on your screen is held in place by a plastic bezel. This bezel holds the metal tabs that secure the entire assembly to the phone. This plastic bezel's glue has failed causing the screen to "pop" when it is pushed back into that bezel.
Some people receive their phones with a bad bezel, while others have bad batteries that expand pushing the screen away from this plastic bezel. This will typically occur on the left side of the screen. (So, if your battery isn't holding a charge and randomly fluctuating this might be a contributing factor)
If your phone is under warranty, apple will swap it out for free. If not, you will need to pull your screen off, and re-glue that bezel to the glass. I pulled mine away from the screen and used a tooth pick to "paint" super glue to the edge of the exposed screen. Works like a dream.
Keep in mind if your battery is bad, you will need to replace it before gluing your glass.
Important note: self repairs void your warranty, but if you're out of warranty it doesn't really matter.