your reset button is not a submit button, it is regular button that you are using javascript to reload the page with a URL variable:
<input name="SearchReset" class="btn" type="button" onclick="window.location.href='sponsor_results.php?reset=true'" value="Reset" style="width: 90px;" />
in the clear session behavor though, you are using a post element for the trigger:
if (isset($_POST["reset"]) || isset($_POST["reset"])){
it should be the GET element:
if (isset($_GET["reset"]) || isset($_GET["reset"])){
since clicking the button does not submit the form.