I realize this is an old thread, but I have exactly the same issue in Safari, and only in Safari. I just got a new iMac with Mountain Lion installed, and am using the wireless mouse that came with it (whatever they are calling the mouse these days). If I'm on a page and merely move the mouse over toward the side of the page, it suddenly will zoom up one or two magnifications.
I know how to "double tap," and I'm not double tapping when it happens. I'm not lifting my fingers on the mouse at all. When this happens, double tapping sometimes will take it down a notch in zoom, sometimes not. I just do Cmd-zero to return the view to normal, but it's extremely annoying, and apparently Apple is comatose to the problem.
I've searched everywhere trying to find out why this happens, to no avail.