Since you mention this happens when you turn up the volume, the likely reason is that vibration is causing one or more of the FX control knobs to 'wake up', which would change that FX to whatever it is pointing to at the time.

One way past this is to 'park' the controls at '0' (counterclockwise all the way), then select the preset sound you want.  If this happens when in manual mode, you can save the tone you have adjusted to to a user preset, then park the knobs at 0, then select this new preset.

Someone also suggested putting a small Oring under the knob so it does not turn as easily.