I noticed that when we have certain characters on the text field in a DropDownList the control replaces by '?' the visible text. Then when we submit he compares the value the is stored in local variable and detect that they are diferent wich causes trigger the OnSelectIndexChange.
For instance, I have the value CHILDREN'S RIGHTS but when rendering the control replaces by CHILDREN?S RIGHTS.
Thus CHILDREN´S RIGHTS is diferent from CHILDREN?S RIGHTS, wich I think will cause to trigger the SelectedIndexChanged event
I'm using version 2009.3.1314.35, i don't now if this issue is corrected in newer versions.