The shipping "rules" are actually functions. "calculations" are just like any other column except that they aren't set by adding items to the cart they are defined by formulas... they are recalculated any time any column is updated which could effect it.
Any shipping or tax rule will callculate on demand when it is requested. So they aren't really fired with the cart load... they are fired any time you show them on the page or any time the GrandTotal is calculated.