When using a custom form containing a ListBox control, Outlook unexpectedly adds a new possible value to the control.
Do you know....this behavior is due to the storage limitation of Keywords fields. If the control is bound to a field of type Keywords and you select a number of possible values in the control, the field may exceed its 255-character maximum length. This causes the last value to be truncated. Because the custom Keywords field now contains a new value that is not one of the options in the control, Outlook adds this new value to the list of possible choices in the control.
Please view this useful MSDN KB Article. It has the information about the issue, its cause, re-solution for this issue