Alexa is always listening when you say the wake word. Sometimes it thinks it hears the wake word and responds. Like recently, when I did not say the wake word at all when I mentioned an iPad in a conversation, and Alexa piped up and asked, "Do you want me to order you an iPad?" I answered, NO!
I just figured it was Amazon always looking to make a few bucks any time Alexa thinks it can get away with it!