It's soft grouping and ultimately down to the order of the stack in both the MouseOff and MouseOver layers. What I found confusing was that in your popup_layers.xar file, the first item in the stack is 'Text line' but I can see now that as that object does not have a web property link so Xara then takes the next object '1 Group' and can see that does have a web property link attached to it, therefore it looks at the first item in the stack in the MouseOver layer, soft groups both the MouseOff and MouseOver objects and displays the mouseover effect.