iPhones and automatic 911 calls
As far as I am aware, there is no function to automatically dial 911.
However, anyone with possession of an iPhone can dial an emergency call from the lock screen without unlocking the phone. 

Whether that's more likely than you dialing and not remembering it (whether amnesia immediately following a trauma, or shock from the impact to your body or head is hard to guess as an outsider that wasn't there.