-9 reduction
To answer your question all of the above answers are correct to some degree, but it is a little complicated.
A couple of years ago the plan was to keep all of the -9 until they came upon the heavy C checks and they were going to be systematically pulled out of the system. Unfortunately, there has not been a viable replacement for the airplane except for the E-170/190s which is another story altogether. However, in the last year or so the AD for the airplane came out that really put a dent in the pocketbook so MGMT was trying to figure out what to do. Luckily not all of the aircraft were coming up on the AD anytime in the near future so the fleet as a whole has several years left in them. Recently the mechanics have been in negotiations and that has really sparked a war between AMFA and NWA.
The rumor of the DC9 grounding is probably coming from the fact that NWA has decided to remove 30 aircraft from service later this year. There were many reasons for this as mentioned above, but the primary one was to put pressure on the mechanics. Because of the removal from service of so many aircraft they can now layoff over 600 mechs. Not 1 pilot or FA was furloughed due to this. So as you can see it was just great timing and opportunity on the part of NWA mgmt. No other significant retirements are expected in the near future although there is a plan.
Now this is where it gets really sticky. The obvious replacement for the DC9s are the E-Jets. NWA ALPA continues to negotiate the rates for the SJs. NWA wants to give them to PCL, but NWA ALPA knows if that happens then there will be hundreds of furloughs at mainline along with even more downgrades. To date NO agreement has been reached. Hopefully we will see a resolution to this in the near future..
Hope that helps..