Does the history of the light say anything about it turning on and off? If so, that should tell you what triggered it. But that is an odd one. The only similar thing I’ve experienced is when automations are triggered on “to: on” then if the device goes from “unavailable” to “on” it triggers. I’ve seen this problem when restarting HomeAssistant and every device goes from “unavailable” to “on”. (I solved that by including “from: off”.)