Hi Bob,
I'm not ware of any such field.
As RTI is time triggered rather than event triggered, it doesn't need a reason. It just provides the same set of data every month.
The content of some of the data fields or the fact that some are filled or empty at all can, of course, indicate certain events. But I am not aware of one special field to indicate that event.
You can see the HMRC data items here:
http://www.hmrc.gov.uk/softwaredevelopers/rti/data-item-guide.pdf
hope this helps.
cheers
Sven