This is a tricky point! But short version, the book's answer is the correct one.
English has a class of verbs called "ergative verbs". "Increase" is one of them, along with "break", "turn", "shake", and several others.
When these verbs are used with a single noun, the thing being increased/broken/turned/shaken is the subject. But when used with two nouns, the thing being increased/broken/turned/shaken is the direct object. (Compare "the vase broke"/"he broke the vase".)
Latin doesn't do this. So when translating the one-noun versions of these verbs into Latin, they have to go into the passive voice.
In other words, augeō doesn't mean "increase" in the sense of "the danger increased", it means "increase" in the sense of "the general increased the rations for this week". The subject is always the cause of the increase, while the object is the thing that's getting bigger.
(Note that sometimes augeō was actually used to mean the subject getting bigger of its own accord. However, this wasn't common, and some grammarians considered it incorrect: a -sc- verb was generally preferred instead, such as augescō or crescō.)