Why combobox will select a string automatically after I use showdropdown(true) in my code?

Because this is common practice for combo box controls and a user expects it to be so:
The list item that matches the text in the edit field will be pre-selected.

If you want to select another item or use no selection, call CComboBox::SetCurSel[^]. But note that this will also update the content of the edit field.