If you long-press the action button on the headphones that usually triggers the voice command, and if your voice-control app is Google, then that's what pops up. Also, if the headphones are using a different electrical signal standard (i.e. Nokia-specific or iDevice-specific), they might be triggering the voice command with a single press or any other action that wouldn't prompt it otherwise. Of course, a short-circuit in your headphone's control box might also trigger it, but chances are that if this is the case, you'd also have bad audio in one or both channels.