Banned or Bug?

This board feels ominous. Never thought I'd post something here, but anyway.

So, Yesterday I had played pretty much all day. I don't remember doing anything wrong. In fact, it was one of those days that I felt like joking around with everyone. So the day ended and I was off League for about 12 hours. Just a few minutes ago, I went back to my PC (Which I had left with the LoL Client open last night.) And read that I was banned.

I try not to panic, as I've spent a lot of time on my summoner account. So, I just figured there was a bug like the time I received a new summoner icon (Reward) and was logged off in order to receive it. But no. I enter my password, and the following message pops up on the screen.

"This account has been suspended until January 5, 2016. Please check the email address associated with your League of Legends account for additional details."

So, I pretty much got banned for a few hours? How does this work? I'd like to know what I did to get banned in the first place.

Here's a screenshot of the ban screen.

EDIT : I found a more informative email in my spam section. The ban is justified. Sorry for all your troubles.