This probably has nothing to do with set tab order.
Enter layout mode and select the field. Check the behavior settings section of the Inspector's data tab and make sure that the check box for Browse mode is selected. If it is, see if you can select "ungroup" from the arrange menu while this field is selected as it may have been formatted as a button.
Also, if you get an error message when you click the field that the field is not modifiable, the field has been defined as calculation field.