A few days ago my iPad Air 2 registered a usage jump from 4.5 GB to 10 GB. I talked to a customer support person and they said there was some sort of mistake and fixed the issue. Yesterday, the problem happened again and the customer support person said I used 5GB in a 260 minute session and you're SOL. My wife uses her iPad for Facebook, email, and browsing. She did not watch any videos - not that I don't trust my wife, but we don't get enough bandwidth for streaming - it takes several seconds to even render the home page for google searches - so I'm really sure she didn't use up bandwidth on videos. Our system services including hotspot register less than 300 MB of usage this period so far, so it is not a background task - unless there are mystery unmonitored tasks going in iOS. This unexplained usage spike stuff doesn't work for me because I need a reliable system, also note (if it is a helpful clue) that my system says I've got 14.9 GB of usage on a 10 GB plan. While I'm annoyed that I basically lost 1/2 of my 80 dollar plan I am very, very grateful for pay as you go.
Can you look on the iPad air at the usage? On an iPhone it's under settings>cellular and you can see what used so much data. It could be something got stuck in the loading or sending position.
Keep in mind if you're looking at your bill and using the time stamps to figure out when data was used, that won't be accurate. Data gets reported randomly so data you use at say 8am may not be reported until 10pm so you can't ever know exactly when data was used.
There was an Apple update this week.
Your iPad should indicate what apps were using mobile data and the quantity. If the data used this month doesn't match Verizon records, please take a screen shot to prove it.
Make sure to manually close all apps. Just because you return to the home page, your apps will still be running unless you do this. I found out the hard way when I first got my iPad. Once I realized that 20-30 apps were always running in background and turn them off, immediately after using, my data usage went way down.