The pic of the trailer shows the door opening from right to left, meaning into the wind. Maybe the door wasn’t locked properly and wasn’t sealed, so in her attempt to slightly open and reclose the door, it caught the airflow and ripped it open. That much suface area on the door would easily be enough to pull her out when it opened.
We cannot know about her actions, but in our Airstream, we left the door open somehow— did we follow the check list? The door caught the wind and hit the catch hard enough to spring the door so that a mechanic was required to bend it back to a door that would close.
Interesting, in this condition we drove through a southern check point and no one noticed the open door. (The agent stood on the driver side and motioned us through.)
We never drove with a person or pet in the trailer though, it is not safe.