The WebAnnotationViewer needs to bind to the parent WebImageViewer's ZoomChanged event, setting this to a function of your own instead of using atalaEventAdd will cause it to break all annotation specific operations (such as automatically resizing annotations)

This is the recommended way to attach to this event:

atalaInitClientScript('OnPageLoad();');

function OnPageLoad(){
   atalaEventAdd(this, WebAnotationViewer1, 'ZoomChanged', OnZoomChanged);
}

function OnZoomChanged(){
   // do stuff ...
}