Event is firing for every Row

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XAML

MVVM

DataGrid

Silverlight5

I am using a cascading comboboxes inside datagrid.
i am able to get the datas based on selectionchanged but the that event is firing for every row
Here is my code

<sdk:datagridtemplatecolumn header="Category" width="110">
                            <sdk:datagridtemplatecolumn.celltemplate>
                                <datatemplate>                                    
                                <combobox foreground="Black" height="30" isenabled="{Binding Source={StaticResource EffortViewModel}, Path=ComboBoxStatus}" itemssource="{Binding Source={StaticResource EffortViewModel},Path=ProjTypeTaskCtry}" displaymemberpath="TaskCtgyName" selectedvaluepath="TaskCtgy_FK" selectedvalue="{Binding Source={StaticResource EffortViewModel}, Path=TaskCtgy_FKField,Mode=TwoWay}" />
                            </datatemplate>
                            </sdk:datagridtemplatecolumn.celltemplate>
                        </sdk:datagridtemplatecolumn>

<sdk:datagridtemplatecolumn header="SubCategory" width="110">
                        <sdk:datagridtemplatecolumn.celltemplate>
                            <datatemplate>
                                <combobox foreground="Black" height="30" isenabled="{Binding Source={StaticResource EffortViewModel}, Path=ComboBoxStatus}" itemssource="{Binding Source={StaticResource EffortViewModel},Path=SubCtry,Mode=OneWay}" displaymemberpath="TaskSubCtgyName" selectedvaluepath="{Binding TaskSubCtgy_PK, Mode=TwoWay}" selectedvalue="{Binding TaskSubCtgy_FKField,Mode=OneTime}" selectedindex="{Binding TaskSubCtgy_FKField}" />

                            </datatemplate>
                        </sdk:datagridtemplatecolumn.celltemplate>
                    </sdk:datagridtemplatecolumn>