Page life cycle and property assignment

I have two dropDownLists that have been extended as user controls.  They are part of a group of search criteria the user can select.  One DDL has a list of organizations, the other is a list of sub organizations that is populated based on what is selected in the first.  Each is dynamically loaded from the database.  There is a property on ddlSubOrganizations called 'selectedOrganization' that I use as a parameter to populate it.  So if the user selects 'A' from ddlOrganizations, ddlSubOrganizations lists all of the child organizations that are listed under 'A'.  They're both in an AJAX update panel.  So if the user selects an organization, the page posts back, and loads the sub organizations based on the selected organization. The problem is that when the user actually clicks the submit button, if the value in the organziations drop down is 'All', it sets the sub organizations to 'All' and doesn't mai ...