1. We had the sharpest market drop in history. Models could predict this, but while this drop is say a 1 in 30 year event across all industries, this drop for certain sectors (like airlines) is more of a 1 in 100 year event. If companies all planned all the time for once in a century events, global growth would be much slower.
2. Most companies in most industries are still better for having done share buybacks. We're only back at 2017 stock levels. They've been buying back since 2010. The best return most companies got for their cash was doing this, and I'm not sure that's changed with this downturn. Most companies will be back to normal pricing in a year's time from now anyway, and their share buybacks will still have been as brilliant of a decision as ever.
* Government regulation impacts
* Union strikes
* Aircraft groundings due to faulty (or less-long-lived than anticipated) parts
* Ticket search companies undercutting their profits
* Oil (and thus fuel, one of their main costs) price surges
* Wars and conflicts that decrease travel demand or cause large unsafe-to-fly zones over the world.
* Volcano eruptions that fill regions of the globe's sky with engine-killing volcanic dust.
* Terrorism (the reduction in passenger demand in the wake, groundings while implementing new safeguards, related lawsuits in the aftermath, etc)
* Now-obviously: Pandemics (but even smaller epidemics have shown effects several times in the past couple of decades!)