Though any device can increase its data usage without us knowing, I'd be willing to bet that it's the DirecTV receiver. It is advised against connecting a DirecTV receiver to HughesNet as there is no way to control its data usage. It can, and often does, use data for things other than On Demand viewing, including guide updates, program descriptions, searches and the database for such. I also suspect, though I'm not sure, that the receiver downloads those instant watch movies you sometimes see in the guide and that are normally downloaded through DirecTV's own dish.
Again, I would almost put money on this being the cause of your high data usage. Because of this, I would disconnect your DirecTV receiver from HughesNet. As far as I know high speed internet, though recommended, isn't required for DirecTV service.
If you'd like to troubleshoot where your data may be going, please create a new topic. If you have a satellite TV receiver connected to the service please see my reply to the OP.
I figured this could be what was going on, and it appears, from your post, that it could hit people even more often, and worse, than previously.
Unfortunately, HughesNet just isn't made for the type of internet that DirecTV receivers need. It would be nice if DirecTV would take this into consideration and not act like everyone out there has unlimited high speed internet. After all, the lack of that is one of the reasons some people get satellite TV.
Maybe one silver lining is that it was figured out fairly timely instead of causing you month after month of headaches. In addition, your post may help others to avoid the same thing.