I was told it was system issues between DeltaMatic and PARS.

It was an award ticket that I was on, where NW said that award ticket inventory needed to be available to be able to go on the NW metal nonstop (which there wasn't). DL's SDC rules don't make any difference to what type of ticket you're on - rev or award - if there's a seat left in that cabin (even if it's Y1), you can SDC onto it, no questions asked.