This is called "debt spiral". Its a sign of being increasingly overleveraged. It means even more of future revenue needs to go toward repaying debt. This is a bad sign for a company in an environment where the Big 4 (mainly United and Delta) are massively retiring long-term debt. NK has no levers to pull to increase revenue. They are being crushed by lack of aircraft utilization and "basic economy" that the other airlines have implemented in the fall off of business travelers. The worse thing that happened for Spirit and other LCCs are that Legacy airlines decided to go after leisure travelers and added a bunch of capacity using their massive cash flow instead of financing new planes.

When your beheamoth competitors can buy planes with cash and you are forced to borrow to get planes, you have debt payments your competitors don't have so there is no way to beat them. Especially when you are at a revenue disadvantage without premium revenue seats. Even Southwest is considering adding premium seating to their planes.