Again, here's a link to a thread with a bunch of people with the problem:
https://discussions.apple.com/thread/6542316 It doesn't effect everyone but it also doesn't appear isolated to just certain models of iDevices or carriers, etc. It's iPhones and iPads. It's 4s, 5s and 6s and various models of iPads. It's not all kids phones where we can imply rebellious youth going video-streaming crazy (in my case, I didn't even use my iPad for 3 days and it burned up most of my data sitting in the same (strong wifi) place it always sits when not in use, running on the same plan it's always on, etc).
Everything about my own usage & situation is exactly the same as it was when this same iPad was running iOS7. The ONE change was the "upgrade" to iOS8. If you go read that thread, you'll see that said by many others. And that's just one thread on this topic.
Called AT&T about it and they obviously have had many others experiencing the same problem. They gave me a data renewal because of it. However, they also recommended that I aggressively turn on and turn off cellular rather than just leaving it on as I've done prior to iOS8 (think about that: AT&T recommending turning OFF cellular data ASAP). AT&T giving data away does not imply it's isolated to just a few people. They are not exactly known for their generosity.
As to the OP "solving the problem", we'll see. If you read the thread, there's plenty of entries where others think they solve the same problem by changing things in Settings. And then it comes back again.
I appreciate the "Apple is always right" mentality around here but these various iDevices apparently didn't do this before iOS8. I doubt that everyone posting had a sudden change to their usage patterns... or that their wifi systems simultaneously started failing... or that all of their kids decided to start streaming video in the middle of the night and then denying they did so. What is the ONE thing that DID change for all of those people?