As per my understanding from you above comments, I assume that the SelectedIndexChanged event is being fired multiple times while binding the DataSource. Here, you can always apply below condition to stop executing the rest of the code.
if (SelectedIndex > 0)
Also, set the ComboBox's property - DropDownStyle = DropDownList, instead of any other thing.
And for using SelectedIndexChanged vs SelectionChangeCommited, this below link will explain you one of the scenarios where you might get an error.
There will be more differences (check here)
], but please do let us know if you have any other concerns.