"Sensational News! Mars Attacks! Will Earth Live Another Day?!" But serious, Drupal has been forked. The fork has been called "Backdrop". Wikipedia says, that "In software engineering, a project fork happens when developers take a copy of source code from one software package and start independent development on it, creating a distinct piece of software. The term often implies not merely a development branch, but a split in the developer community, a form of schism."
A group of Drupal developers (lead by the self-identified "Founding Forker" Nate Haug "quicksketch") has decided, that the current path Drupal is taking with Drupal 8 is wrong, and beyond recovery and dispute. Hence, they have decided to take the last 'good' version of Drupal and start their independent CMS, based off it. The development has been moved to GitHub.
Why it happened
Drupal 8 has been taking the course of some drastic changes - moving into a new framework for the core (Symfony) and for the theming system (Twig) at the same time. there have been drastic changes to accommodate the transition. A certain group of drupalers have become concerned, that Drupal 8 will be harder to develop for, and even more esoteric than before. For the philosophical reasons, they believe, that Drupal 8 is being taken over by the corporate, and is being less fit for the use by the smaller companies and freelancers.
What it means and entails
Generally, forking is a healthy thing, which allows to test possibilities and use cases. If transition to Drupal 8 opens a market niche for smaller businesses to use Backdrop, why not. This is very healthy and evolutionary - species are born, species are filtered with the natural selection, the most adapted to the market needs survives. However healthy, this can be and already is, painful. Drupal has had a good community, that, if fork succeeds and survives, will inevitably be torn in two. So, there is good and healthy in this, and there is also some painful aspect. Luckily, Open Source is not a religion, and no one is getting anathematized and burned at the stake or beheaded. (Fingers crossed).
Will the project survive?
It is not clear yet. Forking a CMS means, that over time, it's core will become incompatible with Drupal as we know it. Though the amount of discontent with the changes in Drupal 8 creates some favorable atmosphere for Backdrop, there are also factors that play against it. Namely, it will be some time before it will be usable and used. Sadly, life of Backdrop depends on how successfully it can take away from the current Drupal community. It needs a certain traction, a certain amount of sponsorship and developers to survive. My own take on it, is that I am not sure if the fork will be a success, but I am generally looking positive on it.
What should my clients do about it?
Nothing. Though potentially Backdrop can be a candidate for upgrading to from Drupal 7, right now there is nothing to discuss and to look at. Right now, Drupal 8 remains the sole upgrade candidate, and it is still a year or more to go before a viable Drupal 8 is out there. It will be clear by then, if backdrop should be viewed as a potential upgrade candidate as well or not. So peace on earth and sleep safe! No end of the world. Just evolution.