Most probably an app you recently installed. Maybe a shortcut or widget, or something that needs to enable WiFi to work. If there is such an app and it's set to show toast, the toast notification can show whenever the WiFi state changes even if the app itself is not the one doing the changing.
So, yeah, look at any apps that have permission to toggle WiFi.
FYI if you don't find the culprit, or another direct solution, and if you have Xposed installed, there is a module called Untoaster you can check out.