With 13 in six months, there has to be at least one month with three deliveries (and FWIW eight of those are in the current quarter with five next quarter). I agree that they've probably been doing more poorly at DFW; they really got hit with a double whammy there. WN gained the ability to fly more places non-stop from DAL and added a bunch of capacity into the market, and as a side effect AA was forced to compete more aggressively on price in more markets (they couldn't charge as great a premium for non-stop DFW-LAX anymore, for example). Then AA decided to respond more aggressively to NK with Basic Economy pricing.
UA pulled a fair bit of capacity out of IAH between 2015 and 2018; they dropped by about a million-and-a-half annual passengers from the City's FY 2015 count to FY 2018. That's quite a bit of opportunity for NK.
I guess it's conceivable it's all growth, but I think we saw in the OAG load that NK added frequency in a few markets in that same timeframe already. I guess if it was me doing it, I would want to find some crappy stuff, drop it, and then it means I can put in AUS with a lot of flights and some of it only has to be better than the crappy stuffed I dropped to be earnings neutral. This is exactly how Delta manages its expansions in LAX, SEA, BOS etc. It's more likely to be accretive with that strategy if you are making big moves. Then you can add some onesies and twosies elsewhere in the network that aren't as risky as a huge entrance into a single market where you are unknown.
I know it seems like this is no bigger than what F9 does, but this is all daily. This is equal to F9 adding 27 destinations from a city with their 2/week schedules. This is a huge move for an airline of NK's size. It's like 2% of their fleet. That's going to impact earnings one way or the other.