Voice Dialling opening on lock screen

I've got a Moto G running KitKat and am having an issue where the Voice Dialler (I think that's what it's called) opens up when the phone is in my pocket, locked. I have no idea how it is happening.. I presume there is some combination of buttons you can press to open it, and these are getting pressed in my pocket. I regularly have headphones plugged in to the phone, so it is perhaps related to that. My question is, how is this Voice Dialler app opening and how can I disable it? I haven't installed any third party app so I presume it's a part of Google Now, or some Motorola app that came with the phone...? I haven't been able to get a screenshot, but the app opens and a voice prompts me to say a name or phone number to dial.. Very annoying as it cuts off Spotify and I have to unlock the phone and hit the play button again.