My S4 keeps acting like i am hitting the back button

[AC Question]

I have an S4 running version 5.0.1 It is about a year old. i have been running into an issue that is driving me crazy. For no reason, my phone starts activating the back button without me even touching it. If i have the key touch sound on, you can hear it like i touched it. I have turned everything off, removed battery, removed cards and restarted. I have started it in safe mode (only allows factory apps to work) and it still does it. I factory reset and it will fix it for about a week. What in the world is triggering this? What is the fix if it's a software issue? With the safe mode and the reset fixing it, i can't see how it is a hardware issue. Anyone else run into this and how did you fix it?