Strange Behavior

I have a ListBox whose selectionMode is set to MultiExtened.

Pseudo code to follow.

In MainFormMenu:
Select MenuItem1
CassCertify CC = new CassCertify();

CC.ShowDialog();

CassCertify Form pops up.
Select OpenFile Button on CC Form.
Populate Listbox on form from file.
Select an item in list box
Press ctrl key
CC.ShowDialog() in main form --- Debug stops and errors out.

The question obviously is why is it going back to the Main form and trying
to run the CC.ShowDialog() command again at this point?

Thanks