What causes a "Community Event Buff"?

I just logged in to play, and was greeted with a Community Event Buff (Your Experience gain is increased by 50%):

What causes this? I'm only part of one community, which I created and promptly forgot existed. What are "Community Events"? How do they work?

Solution 1:

The buff has nothing to do with in game community or groups as it sounds,and also it is not repeatable event...or at least until next expansion.

This particular buff is given to all players who log in diablo 3 between 03/03/14 to 24/03/14(Reaper of souls release date) from PC of Mac.

This is part of the promotion of RotS and new 2.0.1. patch. Blizzard basically wants to get as much people as they can back to the game before RotS releases,just to show them what they changed and to get them excited increasing the chances of them buying the expansion.

Here is the official blizzard article,and here is the copy of the same article if you have trouble viewing it.