VoiceAttack by default starts with the last-known listening state, I.E. if listening was suspended when VoiceAttack was shut down, it'll still be suspended when it's started back up, and if listening was enabled when VoiceAttack was shut down, it will still be enabled when it's started back up.
What can override this behavior is to have one of the option on the "Hotkeys" tab of the VoiceAttack options window enabled, and set to anything but "Toggle listening start / stop".
Third-party plugins, and profiles with a startup command configured for it, can also change the listening state.