iPhone 5 Restarts With Volume Up

Yesterday I finished school and decided to turn back on the ringer by flipping the silence switch off. Upon flipping the switch, my iPhone froze to which I then proceeded to restart my phone by holding the power and home button. As my phone turned back on, everything seemed fine until I unlocked my phone and the apple logo was overlaying the home screen. I turned off the screen and turned it back on upon which I was met the ringer overlay box which showed my ringer volume go all the way down and then stay up like I was pressing the volume down button. I restarted my iPhone again this time with silence mode back on and it started up without the ringer volume box, however, the volume buttons were unresponsive. Today, I woke up and went to school and neither of the volume buttons were working at all this morning, however, this afternoon I was watching a video and I accidentally hit the volume down button and it actually worked so I proceeded to press volume up button to test if it worked and upon doing so, my phone froze for about 3 seconds and then a bunch of vertical lines filled the screen and it rebooted. I restored my phone and updated to ios 7, but it has not helped - my phone still freezes and reboots if i hit the volume up button. This is my second iPhone 5. I've taken very good care of both my iPhones - did not drop either of them, kept them both in an otterbox throughout their lives, and never got either of them wet. I got my first one replaced because the power button died after normal usage after about 5 months. Now, I'm thinking Apple might have to replace this one too. What do you think happened to cause this? Is there anything you guys recommend to keep something like this from happening again since my warranty expires in a few days? Here's a video of the issue: http://youtu.be/RAs1cjeUWtk