There is some kind of bleed-over from your speakers to your microphone. Whether that is from your speakers being too loud, your mic being too hot or from a hardware/driver issue, the speech engine is picking it up what it thinks is speech through the device it is listening on. Additionally, if you have a command called, 'all' and your command weight is set high, you could cough and the speech engine would interpret it as, 'all'. What is your command weight set at (its on the options screen)?
On a side note, VoiceAttack wont just randomly execute commands on its own (like a 60s robot sputtering out of control lol). It reacts to events (recognized speech, mouse clicks, keyboard and joystick button presses, subcommand execution, external execution (command line, plugin), etc.) and then executes a command based on those events.