AJAX :: Textbox Clears After A Few Secs In Update Panel?

Strange problem, not sure why it's happening. Basically I have inside an UpdatePanel a dropdownlist, a textbox, and a regex validator. page loads, ddl is bound and then based on the value in the ddl, a different regex is applied to the textbox. DDL could have search by "date" (with date regex), "last name" (with az regex), or "zip code" (with a zip regex). This way one search box can be used with 3 different regexes. Select zip code in the ddl, the OnSelectedIndexChanged is fired and the new regex is applied to the textbox. Simple.

The problem is, though, is that the page loads, I select Zip Code in the DDL, type in a zip code and then tab out NOT clicking the search button, and as soon as I do the regex validator fires with the red *. I wait about 2-3 seconds and it appears there's a postback that's fired somewhere because the textbox gets cleared out. Here's the update panel.

[Code]....

There is nothing in the code-behind that causes any postback except the dropdown list that applies the appropriate regex to the textbox.I see no reason why after 2-3 seconds the textbox gets emptied out. Is there a way to stop this?