Ninth planet - what else could it be?
What else could it be?  Very simply, it could all be a coincidence...

What they have done is noted that objects in the outer solar system have their orbits grouped in interesting ways. Someone suggested this might indicate there was a heavy planet (or core of a giant protoplanet) that influenced their orbits in this way; the recent announcement is the result of examining this idea in detail.  It is likely to be caused by gravitational influence of such a body.

Of course the TV and newspapers have picked this up and said a planet HAS been discovered. But there might be nothing at all; the loose clustering could all be a coincidence - though that seems, statistically, unlikely at the moment.

Here is the paper for anyone who wants to check.  (Beware, lots of mathematical terminology.)