À larger than normal update, wi-fi assist kicking in more than usual if you have it on, websites with more than average sizes could have all contributed. Take a look at cellular data use to see where it went.
I had the exact same problem...I do not know why it happened. What time did the usage strike in your case? Mine is 8PM EST and I am 100% sure I was NOT using my phone at that time since I was in a meeting.