iPhone 6S Plus Volume randomly changes

Hi, I have had my 6S Plus since launch day but recently I was listening to music through the EarPods and when I put the phone in my pocket the volume suddenly went all the way up and I could not turn it back down, the only way I could fix this was to restart the phone. This has happened several times since I have had the phone and even when I remove the headphones the volume still is stuck on full until I restarted the device, this has never happened to me before with my 5. Does anyone have any ideas of what might be causing it, I have checked all the buttons and none of them are stuck in and it has not been exposed to water.

I am running iOS 9.1.