I recently tried booking a group of extended family wanting to fly SLC-CUN, and the lowest fare buckets available on the outbound and inbound were K fares. And since there so many of them (13), I couldn't book them all together, and ended up having to try to get some K fares and some Q fares. Needless to say, it was pricey. And these weren't close in, these are for a Wednesday in February. That A320 is already about halfway full, 2/3 of a year away. DL can and will sell those higher fares on any flight they think they can. And, no surprise, they'll probably sell out (on any flight that has higher fares available).