ASPxButton's click event problem

Question

There are 2 combobox controls A,B and a button control C on the page.

B has a client side SelectedIndexChanged event that invokes

the method A.PerformCallback();C has the click event on the server side and its autopostback=false;

When i click C on the page ,its click event occurs first normally. But if A.PerformCallback() is executed first (select a different item from B),then click C after that ,you'll find A's callback method runs too before C's click method runs ?Why will A's callback method run after C has been clicked?