Here's why:
* Intransitive verbs do not take a direct object. They express an action that is complete in itself.
* Transitive verbs need a direct object to complete their meaning. They express an action that affects something else.
In the sentence, "tumbled" describes Angela's action, but there's no object that she is directly affecting. "Down a steep slope" is a prepositional phrase that modifies the verb, not a direct object.