Scientists weren't quite sure what caused this, so they started running computer simulations of different things that might make them clump in the observed way. In January 2016, they published a paper presenting a potential cause for the phenomenon: a hitherto undiscovered giant of a planet, 10 times the size of Earth, skulking all but silently about the edges of our solar system like a cosmic Hodor.
Arguments for and against the theory are ongoing and just so, so dorky -- but computer simulations certainly do support the idea that a large planet could be causing these groupings to occur. So we might have a whole new planet! Your outdated knee-jerk answer of "nine" when somebody asks how many planets are in our solar system may once again be correct!
As for everyone who grew up with eight planets: Suck it.
How It Could Destroy Us:
If there's a previously unknown ninth planet, Earth could be looking at a surprise centaur invasion.
Disappointingly, not this kind.