Really, when a department seeks an extension past 30 days, it should be able to demonstrate that there are a very large number of records or that the scale of the research involved is so significant that it's disruptive to operations.
Consultations with other institutions or third parties implicated in the records would be another significant contributor to requests for extensions.
In the case of TBS as a central agency, for example, I know that in my own work there is an awful lot that we do with third parties that are partners in many of our efforts, such as provinces and territories and other external bodies.
I don't know the specifics of TBS as a department, but I would anticipate that some of the extensions that are required are likely associated with third-party consultation.