Usage that high is some form of data sharing or a particularly large family. As an example, 3 adults in my house streaming movies/Netflix, twitch, uploading to youtube and gaming, basically 24-7 is roughly 350-450 gb monthly, with occasional spikes when downloading a new game that's particularly large. I'd start with making sure you don't have someone in the household downloading/uploading entire libraries of games, movies or something else, as that can add up quickly. I would also assume every streaming user in the house is going to be consuming between 150-200 gb each.
Otherwise your biggest hitters are running a website/server, network access storage or any equivalent style downloading/uploading, BTC farming or resource sharing is another big one, and all of the above but due to a nefarious program/virus or illicit access to your wireless.
