Right now tag pages only accept Latest sorting by default. Hot and Top sorting are also available, but they must be selected manually by users. There is no way for admins to set them by default (right?). See for instance tags.
The motivation to allow sorting tags by Hot by default is the same as in Allow setting "Hot" as default topic list for categories. The solution could be a setting just like there is one to decide the sorting of the Main page. Or an alternative / stepping stone would be that Tags sorting just follows the Main page sorting, and if admins have set Hot by default in the Main page, tags would be sorted by Hot as well.