You sound like you’re running afoul of Alexa Hunches. Had it happen to me a few weeks ago with a plug. clicked it off and on three times then a few minutes later shut it off. I’ve also seen it dim a light then [Mod Edit] it off a bit later.
You can check if that’s what happened by going to the devices page in the alexa app and looking for the hunches button next to the guard mode button. It’ll keep a list of things Alexa has had hunches about and ask if it did the right thing.
MOD NOTE: Post edited to conform to the Community Guidelines.