OK just wanted to do a quick update on this...
Just now it happened again and I wanted to be a little more specific. It's not Moto Voice that's activating on its own. It's the screen that comes up when you tap the microphone on the google search bar at the top of the home screen. However, when this does happen, it never stops spinning and then tells you "Say OK Google or tap mic" with the list of suggested commands under neath it. It just keeps spinning and spinning until I notice it and hit back/home.
This isn't the biggest deal in the world, but it would be nice to figure out why it's happening.