Hard to say. Streaming videos uses a lot of data for example. Just depends on what you're doing or of any of your apps are data hogs. Have you changed your usage habits? Viewing more videos or anything?
You may do well to check a couple things...
Do you keep a lot of apps running in the background?
Do you allow app updates to download and install via cellular data or strictly wi-fi?
Much the same should apply to your app settings in general which, if allowed to do so, will default to using cellular data instead of wi-fi.
Have you checked your data usage on verizon on your account to see when the data was used (time of day and date) that may give you a hint as to what you may have been doing when the data was consumed. You may also be able to get an app to help monitor data usage in the future (newer Android devices have data usage built-in that tells you what apps have consumed the data too).