When an Event is triggered by a user in IE, it is set to the window.event object. The only way to see what triggered the event is by accessing the window.
From what you're describing, this problem is likely a result of the unique event bubbling model that IE uses for JS.
My only real answer is to ditch the ASP.NET validators and use a jQuery form validation plugin instead. Then your textbox can just be a regular ASP Webforms control and when the contents change and a postback occures all is good. In addition you keep more client-side concerns seperated from the server code.
I've never had much luck mixing Webform Client controls (like the Form Validation controls) with external JS libraries like jQuery. I've found the better route is just to go with one or the other, but not to mix and match.
Not the answer you're probably looking for.
If you want to go with a jQuery form validation plugin concider this one jQuery Form Validation