P-90s hum. In fact, ALL single coil pickups hum, but Fender style single coils are quieter, so hum less.
Hum is caused by appliances (refrigerator motors are an example), lighting, amps, TVs, pretty much anything using AC current, as the alternating current affects the magnetic fields of the pickups. House wiring can be enough when appliances are on.
As Justmund says, they are louder than regular single coils, so they hum louder. This isn't unique to PRS P-90s, it's part of every P-90 guitar going back long before PRS got into the game. My '65 SG Special with P-90s has plenty of hum.
In fact, this is the reason humbuckers were invented way back in the early 50s for guitar pickups (there had already been humbucking coils for microphones 20 years or so earlier). The pickups are wired 180 degrees out of phase, and this causes hum cancellation (and also affects the high end of the pickups to a degree).
As you probably have realized, you can turn in a circle and find a spot where the hum quiets down. This is sometimes called "facing Mecca" by P-90 aficionados. I love P-90s, they definitely have their own cool thing happening, but they do work best if you find the right spot to play in.
