Textbox on user control causing a redirection to default.aspx every other firing

I have a simple search user control that consists of a textbox.  The textbox's onchanged event appends the entered text to another page address and response.redirects to that page.  The target page reads the query parameter and displays the results.  Works great the first time a search term is entered.  The problem comes if I immediately enter another search term, instead of refreshing the search results page with the new query parameter, it jumps back to my default.aspx page.  I'm not sure where its getting that as an address to redirect to, the only control with default.aspx as a target is a imagebutton on my master page.  I've tried turing off the autopostback on the textbox but it didn't help the problem.
Here's the search user control
<%@ Control Language="C#" AutoEventWireup="true" CodeFile="SearchBox.ascx.cs" Inherits="usercontrols_SearchBox" %> ...