XL2003 on XP

I have a UserForm set up as a splash screen that fires with WorkBook code:

Private Sub Workbook_Open()
Userform1.Show
End Sub

Sub KillForm()
Unload Userform1
End Sub

And the code behind the UserForm:

Private Sub UserForm_Initialize()
Application.OnTime Now + TimeValue("00:00:06"), "KillForm"
End Sub

If the user kills the splash screen before it times out, and then closes the
application without closing the instance of Excel, the application will start
up again after the selected time out period. After that, even if the splash
screen times out, this repeats when you try to close only the application.

Can anyone explain what causes this effect?

--
Al C