what causes it?

There is no one answer because RTN is a kind of symptom, albeit one that is very dramatic. One commonly implicated cause is low alkalinity.

Stress.
could be a cause by a range of water quality deficiencys such as High Phos or Nitrate and low Alk and Mg.

If you get it do all of your SPS die?

no. usually its only isolated to specific corals but in some instances if the stress is great enough then all the SPS will just RTN.

hmm ok thanks. i have never been able to keep SPS alive, even with perfect water quality
they all get RTN by the first week or so...

I had a softball sized Birdsnest that RTN'ed out of nowhere a month or two ago. It had been growing well and getting super bright pink for about a year. Luckily, I was able to frag three 2" chunks off that have re-grown nicely.

Sorry to hear about your RTN, it's such a drag :/

i have lost a big green birdnest, rainbow monti cap, pink birdnest, green slimer, and other various montis and acros

That's strange about the montiporas and birdsnest. I've usually found them to be hardier than acroporas.

something must be wrong with your tank then.... do me a favor and put your pinky into your tank and see if you feel anything weird...