it does not happen here ?
I dropped a TTreeView on the form, added a Parent and a bunch of children nodes.
Placed a button on the form and when run, I select one of the children nodes so that it's selected and then depress my button which looks for the selected node in the list, it finds it, and then deletes it.. I get no onclick trigger, only when I actually click on it.
Maybe its a widget problem ? I did this test in windows