Button Click changes active form

I have a Main form which loads a customer management form with listview for
customers, Add, Edit, and Delete buttons. Add & Edit events load the
customer edit form.
On the customer edit form I have a tab Control. One of the tab pages
contains two listviews. One for ALL available items to associate with the
customer and second for those items selected to associate.
For now it simply removes the list item from one listview and adds it to the
other listview.
My problem is when I click the buttons to move listview items back and forth
between the two listviews the code runs as expected - but for each click the
focus gets returned back to the management form instead of staying on the
edit form.
Nowhere in my code have I manually set focus on anything. Why does the
button click change the form focus?