Hi Megan,

"secoué" is an adjective relating to the subject "On" where "on" is, in this case, more than one person.

We were not shaken/agitated all over the place  --> On n'est pas secoués dans tous les sens.

The past participle "secoués" is acting as an adjective

Nous vs on (subject pronouns)

Hope this helps,

Jim

Would "on" always be considered as multiple people in this context to a french ear? Could it also be considered as a more general "you're" or "one" like it would in english? 

On can stand for a colloquial "we" -- in this case it requires the plural in adjectives and participles. Or it can be a more general "one" or "mankind", then it needs the singular. Here it is the colloquial version of "we" and therefore needs the plural. There's a lesson on this here: On can mean either we/one/people (French Subject Pronouns)