This is not VoiceAttack changing your mic volume, but rather the Windows speech engine. The reason the speech engine does this is to compensate for the microphone state when you had trained it. So, if your mic was too low when you trained it, the speech engine will raise the level. If the mic level was too high, it will try to lower the volume.
You will need to retrain the speech engine and try to keep the mic level in the green.
Note - a band-aid is provided within VoiceAttack in order to get around this, however, it is just a band-aid and your results may not be desirable. To prevent the speech engine from changing your mic volume, you'll need to go into Options > System/Advanced and check the box, 'Prevent speech engine from changing microphone volume'. Again - this should only be used in a pinch and you should retrain your speech engine for best results.
Hope that helps!