Session_end event raised even open a different browser? vs2010,webform

Question

void Session_End(object sender, EventArgs e) 
{

    HttpRuntime.Cache.Remove(this.Session.SessionID);
}

when enter into debug mode(a break point has set on the above code line), VS brings up an IE page, copy the url&paste to another browser of different type and hit enter, the break point is hit several times,why?