When certain iOS devices are factory reset, they default to cellular data being turned on (and available using Apple-provisioned carrier credentials) to use mobile data service to activate even without connecting them to a local network.

I know at least a cellular iPad Mini has done this to me when I didn't want it to. (I was doing an experiment for a year where I try to keep Apple from knowing my location, via Wi-Fi VPN routers and no location services. FWIW, the experiment failed because of surveillance features like this.)