Textbox postback interrupting form submit (with validation)

I'm having a small issue with page I am working on. I don't fully understand whats happening and was wondering if someone could throw some light on it.
 
I have a text box that causes postback, on postback it does some calculations on the textbox value and updates some lables on the page. Further down the page I have a submit button that takes the textbox value (and some others on the page) and runs a process to generate a file using the values. The textbox and submit button belong (along with some other controls) to a validation group that just makes sure the textbox and other values are not empty when you try to submit the form (to generate the file). The problem is when I change the textbox value and then try to submit the form, it seems to fire the function for the textbox value changing (and updates the labels) but it does not call the function for the form b ...