I haven't been on on a while and was reading the news. I saw an event about splashed packs? What is this event and how do you play along? I searched, but couldn't find anything else about it.

It just ended today. :)
Basically, you buy water balloons from the news page and throw them at packs on their pages. If you hit them by chance, your allegiance gets a point. The winning allegiance (in this case, Neutral, duh.) gets 200 mushrooms at the end of the event.