Large download on cellular ( 5.5gb ) while I'm sleeping, and phone is connected to wifi

Last night my phone downloaded ~5.5 gb on the cellular network. We are connected to wifi, and coverage is rarely ( if ever ) a problem. Why would my phone have grabbed 5.5gb at 1 am?

Has anyone else experienced this?

Unfortunately, my mobile data appears to be lifetime, as it shows 200GB of cellular data, and many apps are over 5.5 gb of downloads. Although it is hard to imagine any of my personal usage being the reason. I've only used 8gb of youtube, for example, the entire time I've had my phone. For me to have used 5.5 gb in one night seems very unlikely.

Any insight appreciated