event triggered on post back

i have a user control
in which i am dynamically creating one drop down list control and some other controls
i have a static button in the user control.

i am attaching an function as selection index changed event

it is triggered
when i change the selection in that drop down list
and also when the click the static button

when i click the static button, i triggers load event of the page ,
call the function which is assigned as event for drop down list
before calling the click event function of the button

i am using visual studio 2010 and .net 3.5 framework