I think that this is another candidate for event delegation rather than an Node.on.
The wire function is called for each expandable branch and just sets up calls to ajaxLoad if the node is expandable. It should be possible to determine whether a node is expandable in the ajaxLoad function and instead to delegate to all tree items.

I'm not familiar enough with the code to do it right now, and I know that you're already working in this area Sam so I point the finger of doom at you