Thanks, this does sound reasonable. Maybe add a tooltip or a textual paragraph somewhere on the page explaining the shift-click thing then, for those who wonder how to get back to the default sorting (or even extend it to three columns, or something).
I think most people, even when they wish to use a T-first sorting, just see it’s not possible and continue on living, instead of putting up a feature request and nagging the site admin, as I do ;-) Maybe that’s why you think it’s the common case.
I do agree that a D ASC, T ASC sorting should be the default.