What gesture causes the page to zoom in?
It appears not the be the case for the Magic Mouse, but with the trackpad, there is a pinch to zoom gesture also. If another finger were touching the trackpad, one finger's movement could be interpreted as a pinch.



Another option is that some part of your hand is coming in contact with the mouse surface and is being interpreted as a double tap. If you turn off the double tap gesture in System Preferences, does the problem persist?