NOTE: This is an unconfirmed report. But given that it's from the AP, it's worth noting. Stay tuned.
The satellite is damaged, and cannot maintain orbit. In low Earth orbit, the atmosphere is extremely thin, but over time even that can slow and degrade a satellite's orbit. The spysat's orbit is dropping, and that's bad. When this happens, there is no control over where it might fall, and that means it could drop potentially hazardous debris over populated areas.
The chance of that is extremely small, but in an effort to minimize it, the AP reports that the US Navy will be used to launch a missile to blow up the spysat.
Now let's be careful here. Unlike the way the AP article phrases it, you can't "shoot down" a satellite; it's not an airplane being held aloft by its wings. If you blow it up with a missile, the parts will continue to orbit the Earth (which is why the Chinese angered everyone in the space industry recently when they purposely blew up one of their satellites, sending debris into orbit that will potentially threaten other satellites for years). Imagine driving behind a truck with gravel flying off it and you'll get this picture.
However, the advantage is that smaller pieces will burn up more readily when they finally de-orbit. So blowing it up is an option, if done very carefully. If this is done a few days before the satellite makes its final descent, for example, the debris won't be in a place where it can harm other satellites; by that time the spysat will be very low over the Earth, and other satellites will be much higher and out of the way. However, in the last few days the orbit is changing so rapidly that I think acquiring the target with a missile may be difficult. But they cannot do it earlier without creating thousands of pieces of space junk.
We don't have any real info at this time, but when I learn more I'll post.
Tip o' the Whipple Shield to Rob Sparks.