ASP.NET UI field focus backwards after Ajax response?

Question

ASP.NET Ajax, .NET 3.5, IE 7

TextBoxA
TextBoxB
TestBoxC

FACTS:
TextBoxA's keyup event handler causes the Ajax postback. During the postback, user tabs away from TextBoxA.

When the Ajax request is finished, the focus moves to a field they're not on (and didn't ask to be on, really).

GUESSES:
ASP.NET Ajax is tracking which field has focus at the beginning of the ajax request and is working to put focus back on that field as part of the response.

QUESTION:
How can I keep the focus from being set? I want the user to do whatever they like with focus during the ajax request and not have that overridden when the request is finished.

I inherited this code, but I don't see anywhere that I'm setting this focus explicitly...