Because the dynamic is only concerned with the relationship between cause and effect, which in turn determines the path through the model.
The nature of it is inconsequential to the meaning of a story. An Action driver that forces decisions can happen within a Universe context just as much as it can from a Mind perspective.
Drivers are not tied to the context of the Overall Story Throughline—that’s why you’re not asked the “nature” of it, because it has no nature—it’s a dynamic. Not a static appreciation.
Action in Universe context—Kingpin rips open a whole in the space time continuum forcing Peter B. Parker into deciding whether or not to fight him.
Action in Mind context—A Southern racist beats his wife for opening her mouth in Mississippi Burning forcing the Federal investigator on the case to decide to take the gloves off and fight dirty with dirty.