Why is ASP using the code behind from the previous page?!?

I have a chart i am working on and when you click on it (i am working on the next page for a drilldown).
I get an error from the server referencing the code behind from my first page default.aspx.cs when it should be running DD1.aspx.cs
 
ARRRRGG!
here is the link >>>> Chart1.Series.["Series1"].Url = "DD1.aspx?SLT=#VALX";
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