I'm not sure what triggers it... but when I'm in my car, Siri reads things back to me. So I don't know if it applies equally to bluetooth headsets or hands-free mode in my car. I don't know enough about Bluetooth profiles to say without some extra Googling.
"Your message to PERSON says SIRI-MANGLED-MESSAGE. Would you like to send it?"
You can send or change it by voice.