Sometimes this happens with me, when I have my Headphone or Apple earphones (just temporary) suddenly the Google Now pops up and activate the mic, and sometime this happens when I'm listening to a song or podcast, and it's really annoying...
Looks like that this happens when I rotate the phone or when I shake it or do some fast movement, and when It's on the lockscreen, the Voice Dialer pops up instead of Google Now.
I don't know if it's the headphone problem (It's the old Apple Earphones, from my iPod, so it doens't have mic, and my Headphone don't have mic aswell) because I never noticed this when I was using my Sennheiser CX 300II (also don't have mic) that died recently.
Anyone know why this is happening and how can I solve this? I bought a new earbud but it didn't arrived yet, but I'm worried if that thing continues to happen.