The sudden rise in fuel prices due to the Iran war seems to be at blame. If you have thin margins and didn't hedge, then a shock like that can wipe you out. Probably won't be the last.
It might've been the straw that broke the camel's back, but they've been unprofitable since the pandemic.[1] Mainline airlines introducing "basic economy" fares that weren't as no-frills as Spirit seems to have been a contributor.
‘Didn’t hedge’ because they set the price below the resilience floor and couldn’t; or because they paid out the good times to shareholders instead of resilience; or..?
[1] https://theaircurrent.com/airlines/spirit-airlines-trump-adm...