The article says "slowing down" but all that I have heard about was mysterious ACCELERATION. Maybe negative acceleration? (deceleration?)
In physics any force acting to influence the motion of a body is called acceleration. There is no deceleration - just acceleration opposite to the current vector of the body.
As for what's really going on.... good question!
It could be that gravity isn't subject to inverse square falloff all the way, but is close enough to having made it seem that way so far. Probably not, but it must be considered.
Or it could be more friction in the outer solar system than previously thought possible.
Or maybe dark matter within the solar system itself?
We know that something weird is going on with galaxies, as they shouldn't behave as they do given the laws we know. Perhaps this is evidence of that same kind of behavior on a much smaller scale?